By selling two machines and using the third for MAME, you can come pretty close to breaking even and getting a free MAME machine out of the deal. It all depends on what parts you already have and how much you plan to do.
The NeoGeo and SF2 will fetch better money, and it sounds like the Combatribes is your best bet to butcher, anyways. It's a no-brainer. As cool as having 3 machines would be, you'll have the other two emulated in which one you decide to MAME. So it would be redundant.
Of course, they're your machines...