Never keep less than six discus in a tank unless they're a proven pair. Sometimes you can get away with four or five, but aggression still tends to be an issue. Discus have a hierarchical pecking order -- the aggression needs to be spread out among several individuals. Three fish is almost always a recipe for disaster. You have two scenarios that usually play out:
1. Two discus gang up on the most submissive fish, which stresses/kills it. Then you end up with two discus, and the now dominant fish will do the same to the new submissive fish.
2. One dominant fish will bully two submissive fish with the same results as scenario #1.
A 55 gallon would be the minimum, but a 75 would be more comfortable.
If you're going to invest in discus, I suggest doing a lot of research and getting your fish from a reputable breeder (either commercial or hobbyist) rather than the LFS, which are known to carry lower quality fish and which also could have been exposed to disease/parasites in centrally-filtered LFS systems. This is why most LFS do not offer a guarantee on them.