These would be nice selections for a 75:
-Aulonocara stuartgranti or a smaller Lethrinops (red caps or Albus)
-Copadichromis (just about any of them but borleyi)
-Placidochromis (Jalo Reef and phenochilus Tanzania are my favorites)
Two groups of the above would work in a 75. As a third species, you could also likely add a less aggressive/smaller mbuna (P. salousi, yellow labs or rusties would all work with the fish above).
In general, stick with fish that stay under 6". Try to go with less aggressive species... I'd avoid jacobfreibergi or line bred peacocks. Try to pick very different species to avoid hybridization - no two from the same genus, no two too similar (Scianochromis fryeri, for instance, will hybridize with peacocks).
You'll also want to pick fish that live in different areas of the tank. Most smaller Placidochromis and peacocks will inhabit the bottom of the tank and many Copadichromis will swim towards the top.
