At 2-3" long you'd better hope they don't breed for awhile. If they do they would be pretty stunted. They usually breed when they're about a year old and Discus usually get 5-6" in that time. They do better in larger groups, minimum 4 IME. Larger groups spread out the aggression and one fish typically won't get picked on by the others. With a pair at that size you inevitably get one bully and one whipping boy and the smaller one eats less, gets sick & dies. So by all means add a couple more at least. I'd consider quarantining new arrivals though. If you add more fish you should take a look at your water change regime and make sure you're keeping the tank clean. I've raised 50 fish in a 55g tank from 1/2" fry until they were 4" in size. But I had a large wet/dry filter, a couple HOT Magnums with micron and I changed 50% water 3x a week. So you can accomodate more fish if you maintain your tank properly.

I know have nine discus in my ninty gallon tank, and after the first few days, the pecking order is done..and everyone seems to get along okay. Thing with Discus, is they are a schooling fish of a sorts...