They confirmed one of the themes is Bon Jovi during the Top 14 episode.
My guess is that all three will do both Bon Jovi and Open Choice or Reprise, and then after they do the elimination, the final two will do their winning singles.
Personally, I'd prefer if they do their singles at the Top 5 since they all record them. Plus it's weird to have the F2 do their singles only to turn around and have the winner perform it again like 10-15 minutes later.