That's tough, since most Malawi cichlids won't work in a 3' tank. You could do a single-species rusty tank - some will say you can do more, but there's debate on how "calm" they actually are - or a demasoni/YL tank or a saulosi species tank. That's pretty much it for what's likely to work.
A P. saulosi species tank would look awesome! Blue males and yellow females give you lots of color in one species, and they will be very active!
Another very peaceful mbuna that could work well in a 3' tank is Metriaclima lanisticola. They also have very interesting behavior, since they can be shell-dwelling as juveniles. Unfortunately they are not often seen in stores, but you might find a breeder via a local fish club. They are not as colorful as the saulosi, since all but the dominant male will be predominantly gray.
Metriaclima lanisticola dominant male
Metriaclima lanisticola group
Metriaclima lanisticola fry in front of murex shell