The sources, who spoke in Vienna and from other European capitals, said an informal agreement had been reached to hold the meeting next Tuesday in Paris.
The foreign ministers of France, Germany and Britain would probably meet with Hasan Rowhani, Iran's top nuclear negotiator, the diplomats and officials said on Tuesday on condition of anonymity.
While the two sides had tentatively agreed late last week to meet again in an effort to defuse the crisis, there had been no agreement then on the exact time and venue of the talks.
A senior European diplomat said that by Tuesday afternoon, chances were more than 80% that the two sides would meet in the French capital on 24 May.
Still, further conflict appeared inevitable at the talks.
One of the officials said the three European powers continued to insist that Iran maintain a freeze on all activities related to uranium enrichment.
Since starting talks with the Europeans last year, Tehran repeatedly has insisted that it has a right to enrich uranium and conduct related activities including conversion.
Iran insists it has the right to
enrich and convert uranium
But the Europeans insist on a long-term freeze or a pledge from Tehran to scrap its programme in exchange for technical and economic aid, political support and guaranteed nuclear fuel supplies.
The United States says Iran wants to enrich uranium as part of a covert nuclear weapons programme - something Iran denies, insisting it is seeking to master the technology only to generate power.
Iran last week threatened to immediately resume conversion, precipitating the showdown with the Europeans.
Tehran backed away after the foreign ministers of France, Germany and Britain subsequently warned Tehran of "negative" consequences if it breaks its pledge to freeze such activities during negotiations with the Europeans - diplomatic shorthand for possible UN Security Council action.
British Prime Minister Tony Blair was more blunt, warning that he would support referring Iran to the Security Council if it breaks its word and resumes any enrichment-related activities while talking with the Europeans.
Suspicions about Iran surged after revelations in 2002 that Tehran had run a covert nuclear program for nearly two decades that included development of an ambitious enrichment program and experiments that could be used as part of an attempt to develop the bomb.