The suspects are both dead, and it is understood they took their own lives.
One of the victims was a security guard, according to police, and all three were adult males.
San Diego's police chief, Scott Wahl, said authorities are treating the shooting, which took place shortly before noon local time (8pm UK time), as a hate crime.
Mr Wahl added that the security guard likely prevented further loss of life. He said the suspects, aged 17 and 19, were discovered dead in a vehicle in the middle of the street.
"We do believe the security guard was able to help at least minimise the situation to the front area of the mosque," Mr Wahl said.
"At this point, I think it's fair to say his actions were heroic... undoubtedly, he saved lives today."
The Islamic Center of San Diego, around nine miles (14km) north of the downtown area, is the largest mosque in San Diego county, according to its website.
Aerial footage showed more than a dozen children holding hands and being walked out of the building's car park, surrounded by police vehicles.
The children, who were unharmed, were inside the mosque's school during the violence.
At a news conference, San Diego mayor Todd Gloria said that "no one in our city should live in fear because of their identity, their faith or their place of learning".
Mr Gloria insisted that any hate-inspired violence would be met with the "full force" of local law enforcement.
"Hate has no home in San Diego. Islamophobia has no home in San Diego. We will not stand for it," he added.
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Meanwhile, US President Donald Trump described the scenes in San Diego as a "terrible situation".
Governor of California Gavin Newsom's office confirmed he was being briefed.
His office said on X: "We are grateful to the first responders on the scene working to protect the community and urge everyone to follow guidance from local authorities."
Police understand the suspects died from self-inflicted gunshot wounds.
Additional shots were also fired at a landscaping firm a couple of blocks away, described as a separate shooting incident. Police have not confirmed if this was connected.
A total of 12,391 British drivers were caught driving with drugs in their system above the legal limit at least three times in the 11 years to 20 July 2025, official figures obtained by the RAC showed.
That is compared with 2,553 who received that many drink-drive convictions over the same period.
The Driver and Vehicle Licensing Agency (DVLA) statistics, obtained through a Freedom of Information (FOI) request, demonstrates the need for a national rehabilitation scheme for drug-driving as is in place for drink-driving, the RAC said.
The drug-drive figure is based on the number of DG10 endorsements on driving records on 20 July last year.
These are handed out to people convicted of driving or attempting to drive with a level of a controlled drug above the legal limit.
One driver had accumulated 18 separate DG10 endorsements, while another had 17. Some 56 had at least 10.
People caught drug-driving in Britain face a minimum one-year driving ban, an unlimited fine and up to six months in prison.
RAC senior policy officer Rod Dennis said: "While it's drink-driving that tends to attract most of the headlines, these figures show just how much of a problem the more under-the-radar issue of drug-driving is becoming.
"Drug-driving reoffending rates dwarf those of drink-driving, which suggests current penalties aren't effective in preventing some drug-drivers from repeating their crimes and putting everyone at risk.
"Introducing a national drug-driving rehabilitation scheme - similar to what is in place for drink-driving - could also help drive down reoffending."
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The Department for Transport (DfT) said it was committed to tackling drug-driving in its road safety strategy which was published in January.
This included reviewing penalties and exploring alternative methods for evidence collection.
"Drug-driving is reckless, dangerous and ruins lives. For too long, our enforcement system has struggled to keep pace with the scale of this problem," a DfT spokesperson said.
"But those who do drugs and get behind the wheel will no longer go unpunished. We launched a road safety strategy, the first in over a decade, to save lives by tackling the root causes of deaths on our roads."
The broadcaster said it had removed all previous seasons of MAFS UK from its streaming platforms.
The move follows an external review, commissioned by the broadcaster in April, into the welfare of participants on the programme.
The BBC has reported allegations from two women who alleged they were raped during filming of the show. A third described an allegation of a non-consensual sex act.
They all alleged the show did not do enough to protect them.
In a statement on Monday, Channel 4 said: "In April, Channel 4 was presented with serious allegations of wrongdoing against a small number of past contributors, allegations that we understand those contributors have denied.
"The channel is mindful of the privacy and continuing duty of care towards all contributors, and cannot comment on or disclose details of those allegations.
"Related to those allegations, Channel 4 was asked to respond to claims of failures in welfare protocols. Channel 4 believes that when concerns related to contributor welfare were raised through existing welfare and production protocols, prompt and appropriate action was taken, based on the information available at the time.
"Channel 4 strongly refutes any claim to the contrary."
Priya Dogra, chief executive of Channel 4, said: "I want to express my sympathy to contributors who have clearly been distressed after taking part in Married At First Sight UK. The wellbeing of our contributors is always of paramount importance.
"It would be wholly inappropriate for me to comment on what are very serious allegations made against some MAFS UK contributors.
"Those allegations - which I understand are disputed by the contributors accused - are not something that Channel 4 is in a position to adjudicate on.
"We are also mindful of our ongoing duty of care to all contributors, and the need to preserve the anonymity and privacy of all involved.
"On the claims that Channel 4 may have failed in its duty of care, I believe that when concerns about contributor welfare were raised, and based on the information available at the time, Channel 4 acted quickly, appropriately, sensitively and with wellbeing front and centre."
The show, which is produced for Channel 4 by independent production company CPL, sees single people matched by experts to "marry" strangers who they meet for the first time on their wedding day.
Sky News has contacted CPL for comment.
Officers were called at 2.05am following reports a Jewish man had been assaulted by a number of men outside a property on The Grove.
The Metropolitan Police said its officers were on the scene within six minutes.
They spoke to the alleged victim, who later went to hospital to be treated for facial and back injuries.
The assault is being treated as an antisemitic hate crime, the force said.
No arrests have been made at this stage and police enquiries continue.
The northwest London borough is home to one of the UK's largest Jewish communities, and has seen a number of attacks over the last few months, including an arson attack on ambulances run by a Jewish charity and a stabbing attack, in which two Jewish men were injured.
Iranian Pouria Zeraati was left bleeding in the street after he was stabbed three times in the leg as he walked to his car in Wimbledon in March 2024.
Romanian nationals Nandito Badea, 21, and George Stana, 25, have both denied charges of wounding with intent to cause grievous bodily harm and wounding.
Prosecutor Duncan Atkinson KC told Woolwich Crown Court on Monday that the pair were the money-motivated "hired help" tasked with doing Mr Zeraati "really serious harm".
They allegedly carried out "a planned attack preceded by reconnaissance, and which was ordered by a third party acting on behalf of the Iranian state".
Mr Atkinson said it wasn't a robbery or a fight that got out of control, but a "deliberate" attack intended to inflict "serious injury on Zeraati".
British-based Mr Zeraati worked for Iran International, a competitor to Iran's state broadcaster, which Tehran labelled "a network of spies", and whose journalists were subjected to harassment, the jury heard.
The Persian-language news outlet, its employees and their families, faced threats and in 2023 it relocated temporarily from Chiswick in west London to Washington DC.
As a well-known journalist, Mr Zeraati was "an obvious and readily identifiable target for violence to be inflicted by proxies acting for the Islamic Republic of Iran", according to the prosecution.
His photograph was among a number of journalists who appeared on a "Wanted: dead or alive" poster that was put up in the Iranian capital in November 2022.
The defendants, along with another man, named as David Andrei, who is not part of this trial, spent so long in the area that passers-by thought they were waiting for a taxi, the court heard.
It is alleged that Stana, Badea and Andrei had stayed at a hotel in West Brompton the night before the assault.
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The court heard Badea approached Mr Zeraati and asked for money before Andrei joined him and grabbed Mr Zeraati "with a very strong and firm grip to his front".
Mr Atkinson told the trial: "Mr Zeraati then saw Badea produce a knife from his pocket, and Badea then stabbed Mr Zeraati to the top of his thigh near his buttock.
"He was stabbed three times in rapid succession before the two attackers ran off, leaving Mr Zeraati injured and bleeding."
Stana then drove them from the scene in a Mazda and they caught a flight to Geneva from Heathrow Airport, the court heard.
The trial continues.