I personally wouldnt put discus in anything less than 40gallons. And thats for juveniles. If you plan on keeping then to aldulthood (which I prosume you are), you would want to be putting them into something around the 50gallon mark.
I have 3 juveniles at 3" and a small sub-adult at 5" in my 4foot 54gallon tank and ideally, they would be better off in a bigger tank to be honest (pity I live in a shoe box and have a not so understanding better half, or I'd get a 5/6footer).
G