The source, who asked not to be named, said the visit by Jack Straw, Dominique de Villepin and Joschka Fisher could take place as early as next week if the basis for a deal was finalised.
According to diplomatic sources, Germany, France and Britain have been secretly negotiating a deal with Iran's clerical leaders whereby they would offer technical help to the country in return for full Iranian compliance with the International Atomic Energy Agency (IAEA).
The European Union's "big three" would also insist Iran accept strict controls on its nuclear activities by signing and implementing an additional protocol to the Non-Proliferation Treaty (NPT) that would allow surprise IAEA inspections of suspect sites.
The Foreign Office in London said it had no confirmation of any plans for Straw to visit Tehran and there was no immediate word from either Berlin or Paris.
But a Western diplomatic source here said senior representatives of the three countries had visited Tehran in recent days to "resolve this crisis peacefully".
The Islamic republic, just two weeks away from an IAEA deadline to prove it is not developing atomic weapons, on Thursday said it may bow to demands for tougher UN inspections.
During a lightning visit here by the IAEA head Mohamed ElBaradei Thursday, Iran also promised to accelerate cooperation with the UN nuclear watchdog. However the IAEA director added that Iran had still not agreed to stop enriching uranium -- one of his key demands.
According to the diplomatic source, the German, French and British envoys met with ElBaradei during his flying visit.
The IAEA has also been pressing Iran to sign the additional protocol to the NPT, and provide answers on a number of "outstanding issues" -- notably questions over its uranium enrichment activities.
At an Islamic summit in Malaysia Friday, Iran's reformist President Mohammad Khatami reiterated that his regime had no intention of developing a nuclear bomb but reserved the right to carry on uranium enrichment for peaceful purposes.
He said he saw no reason why Iran could no come to an accommodation with the UN nuclear watchdog.
"We never said we would not sign the protocol and cooperate with the IAEA," he said.
Talks on the protocol between Iranian and IAEA experts are scheduled to begin here on Saturday.
According to the Western diplomatic source, if Tehran accepts all the IAEA's demands, Germany, France and Britain "may favourably consider the possibility of supplying (Iran) with technology, even nuclear technology, as well as nuclear fuel."
The proposal for such a deal was contained in a letter sent from London, Paris and Berlin before the IAEA's board of governors issued its ultimatum on September 12.
In contrast to the EU, the United States has maintained a tougher line with Iran, demanding unconditional compliance and insisting Iran's signature of the additional protocol is not open to negotiation.
But while denying developing nuclear weapons, Iran has maintained that it is being treated unfairly given that NPT signatories are obliged to provide mutual help over civil nuclear technology. The Islamic republic is subject to harsh US sanctions, and only Russia is helping it build its first nuclear power station.
Non-compliance by Iran could lead the IAEA to take the issue to the UN Security Council. ElBaradei is due to report back on November 20 to his council of governors, the IAEA's executive organ.
Washington accuses Iran of trying to develop nuclear arms and has branded it part of an "axis of evil", along with Saddam Hussein's Iraq which it says held weapons of mass destruction, and North Korea, which has said it is developing atomic bombs.