Well the Sparks from some of the 'Cons are not gone so maybe they could rebuild a stronger Jazz in TF3? Like him not being the weakest? A whole new look at Jazz!
Megatron put them all in the back of a stationwagon and hauled them off to be cannibalized aboard the Nemesis.
You're asking us, the ones who couldn't possibly know, how several dozen tons of extraterrestrial bulk are disposed of in a fictional world. Any answer will be pure irrelevant speculation.
I would love that. I dunno, I like the idea of dismantling them and scattering them across the globe and shit.
Convert the dead Decepticons into Soylent Green Energon. Oishii (delicious). Onakagasuita tabechao (hungry, let's eat).