My opinion...
RX-7: These rotaries have a high maintainence cost due to the fact that the rotary engine is not as reliable in those years in production. But it's a rare beauty if you can afford to own one, transmission is relatively weak compare to Supra but it's gear more towards performance road course driving. Be careful when you boost as the turbo is rather weak and watch your plumbing lines...
Supra: Twin Turbo is the way to go, if you get a 5-speed NA and plan to swap in to TT be careful as the swap is not as easy as scoop and drop, a lot of electrical and extra turbo stuff really add to the bill. If you don't like AT, don't get it even if it's a TT, when you boost, the auto tranny won't hold. BTW the Supra is gear more towards shear drag so handling might be a little different, and Supra is hard to drive well as I've experienced first-hand in comparison to the RX-7.
WRX: A beautiful car indeed, amazing in Rally with it's 4WD so it's yet a different kind of performance speed beast. If you like Rally style, WRX is the one for you, when you boost, just be careful and take care of your tranny first as WRX has a even weaker and awkward geared tranny. But my personal opinion say if you plan to boost the WRX, wait for the STi or get the EVO as they have a stronger internal and tranny to start with....
Just my personal opinion as I've driven all three before and this is based on my feeling driving.... Nonetheless, they are all great and I understand it's a difficult choice to make. Whatever you'll be getting, good luck with it!