A senior Whitehall security source told the Daily Telegraph that all three were linked to Isis (Islamic State) cell behind the shootings in Tunisia which killed 31 Britons.
Fallon said: "There are other terrorists involved in other plots that may come to fruition over the next few weeks and months. We wouldn't hesitate to do it again if we knew there was an armed attack planned and we knew who was behind it.
"There is a group of people who have lists of targets in our country, who are planning armed attacks on our streets, who are planning to disrupt major public events in this country. Our job to keep us safe, with the security agencies, is to find out who they are, to track them down and, if there is no other way of preventing these attacks, then yes we will authorise strikes like we did."
Fallon also attacked the "absurdity" of British armed forces being authorised to carry out missile strikes against Isis in Iraq but not in neighbouring Syria. He suggested that the Government was likely to return to Parliament for a vote on formally extending air strikes later this year.
The existence of a "kill list" was denied by Downing Street earlier in the week, but appeared to be confirmed by Fallon.
He refused to say how many jihadists were being targeted, but sources told the Daily Telegraph there were five names.
Cameron approved targets at a meeting of the National Security Council in May, shortly after the general election. The Attorney General was at the meeting and confirmed the legal basis for the action.
On August 21 one of the main targets, Reyaad Khan, 21, was killed in an air strike by an RAF drone. He was targeted after it emerged he was leading a plot to attack the VJ commemoration services in London last month.
Ruhul Amin, another British jihadist described as an Isis "associate", was killed in the same attack.
Three days later Junaid Hussain, a British jihadist who was one of Isis' most notorious recruiters and computer hackers, was killed in a separate US air strike.
Mohammed Emwazi, a 26-year-old from London, was unmasked as Jihadi John, Isis' executioner-in-chief, in February.
He is wanted for the murder of several British and American hostages, and is thought to be in hiding in Syria.
Omar Hussain, a former supermarket security guard from High Wycombe, is thought to be on the list after becoming one of Isis' most prominent recruiters. Raphael Hostey, from Manchester, who is believed to have persuaded dozens of Britons to join Isis, and Siddharta Dhar, a former aide to hate preacher Anjem Choudary, are also likely candidates.
The Government is likely to face significant scrutiny over whether the air strike in Syria was legal.
Crispin Blunt, the chairman of the Foreign Affairs select committee, told BBC Radio 4's World at One: "We need to understand whether the self-defence case stands up in this." He said he would support "a full reference to the Intelligence and Security Committee".
Rights Watch, a human rights group, said it had started legal action against the Government to force it to reveal the legal advice used to justify the air strike.
