I just opened a PMC2 program that I wrote in Simpl a few years ago, and the software allowed me to add a PLX-3, in addition to the existing PTX-3s.

Based on that, I'd say the PLX-3 *IS* compatible with it, because if it weren't, the software wouldn't allow adding it to the program.

Note that this was a PMC2+, not the original PMC2 (without the plus). However, if you already have PTX-3s you most likely have the plus model.

I think SWO's suggestion is the best solution - get an MC3 with whatever remote you want (HR-100, HR-150, or MLX-3) now, then gradually replace all of the remaining Prodigy gear with Crestron models as budget allows.