For the best success keep same type and sub-type fish (same type: african cichlids with african cichlids, etc; same sub-type: peacocks with peacocks, or mbuna's with mbuna's, etc).
Assuming your african cichlids are mbuna's you can add quite a few more mbuna's in your 150gal for a total of two or three dozen.
As far as species selection goes, that will depend on what you want to stock and how much aggression you're willing to have in your tank. I highly suggest accurately identifying the cichlids you intend to get and knowing what to expect from them prior to purchase. Otherwise you could pick up undesireable hybrids, acquire extremely aggressive species, or purchase fish which get too large or may be incompatible with what you already have.
When have an idea of what species you want (based on color AND aggression level), stock the tank with the most peaceful species first, and save the aggressive one's last.
Then balance potential aggression with larger groups of mildly aggressive fish and smaller groups of more aggressive species, for example: acei's and yellow labs, each in groups of 8 and smaller groups (4-5 of more aggressive species). Several females per male will help minimize hybridization (if you intend to breed them) and prevent males from singling out and harassing specific females or specific rival males. There are all sorts of combinations you could go with, the above was just a sample.
If I am to understand correctly the only fish in your 150gal are the three cichlids? Can you post pics of them?