Tony Blair, Michael Howard and Charles Kennedy will be the guests in the live debate on April 28, which will air in a primetime slot on BBC One.
The news follows the Prime Minister's earlier confirmation of Thursday, May 5 as the polling date.
Blair has consistently resisted Howard's call for TV duels like those carried out during the US presidential elections.
The three leaders will appear consecutively in the 90-minute special, each facing a grilling from the same studio audience.
"We did talk about a [head-to-head] debate but it became perfectly clear that particularly Labour had no interest," BBC News controller Helen Boaden told the Media Guardian.
Ric Bailey, executive editor of Question Time, added that an agreement had not yet been reached over who would appear first.
"We haven't been talking to the leaders all that long," he said. "We came up with the proposals and we got a tacit agreement. A lot of the detail and the choreography has got to be sorted out but they will appear live on the same programme."