you can wire whatever ohm speakers together. it will just give you a funky ohm ohm load and you amp might not be able to run at that weird load.It really doesn't matter. But the efficiencies, volume, frequency ranges etc will all be different and the system could be difficult to tune.
To answer your question depends on what impedances the subs are (2 ohm, 4 ohm, single or dual coils, etc) and what the output impedance capabilities of the amp are.
so it isnt good to get 2 12's 2ohm and a 10' 2ohm? there kickers if that helps.dont mix and match different brand/model/size subs.
ok seriously...what are the model numbers?i believe the 12's are 4 ohmi think the 15 is that too