The 3640 should be avoided unless it's very cheap - it doesn't have local RAM, which means the memory access is much slower than other accelerators.
If you want to play 3D games you basically need as fast as possible a computer, at least if you're texture mapped ones. Let's not forget that a 68030/25 is about equivalent to a 386DX25 or so, which is too low a spec for any PC textured game apart from Wolfenstein 3D, which is ...er... rather basic. If you want to just play 3D games like Gunship and Elite/Elite II, then a 68030 will be fine.
The Amiga was never made for 3D graphics, though - or at least, it kind of was, as the blitter I believe has basic 3D functions, but they're very basic and were surpassed by CPUs at about 68020 level or so, I believe. Could be wrong, though.
Either way, if you want to play a 3D game the Amiga isn't really the right platform for it, but if it has to be done get the fastest accelerator you can afford.