ryan is right. three discus is bad. esp in a small tank like that. I think that'd be around a 20gal.
I used to have 3 in a 29gallon. its too small. they set up territories and the two strong fight non-stop. the weakest one never gets to eat. then there's the times when the two stronger discus gang up on the one weaker discus...
Three may work in a 55gal or a 40 gallon though.
If you have a tank that size, i'd go for a breeding pair of discus rather than 3