I've done some omni-wheel research with my VEX kit. It is possible to make an omni-wheel based robot with 3 wheels, 4 wheels, 6 wheels... I guess you could make one with 5, or any other number, but I don't think there are big advantages to doing so.
What Billfred is saying is correct. It takes a little trig to make a 3-sided omni bot work, but it is possible:
http://www.cs.cmu.edu/~pprk/