You should be able to keep something like German Blue Rams in with those and 3ft tank is fine for them as that would give them enough territory to claim. They stay quite small - just a couple of inches - but they are very beautiful fish with fun personalities.
They are very sensitive to changes in water stats though - so you would first need to make sure your tank is in tip-top condition and be prepared to keep up weekly cleaning maintenance on it and water changes, otherwise the Rams will stress and get sick.
Regards - Athena