a 3 foot tank, I heard? I'm gonna have to agree with the no's and advice from soybean, he knows his stuff, and he also knows as well as the rest of us that fish stores and pet stores alike are just trying to sell you stuff, they're businessmen, not experts.
Regardless, it's always better to check around the forums (there are plenty of experts and species enthusiasts on here who can give amazing advice) rather than talking to the pet store clerks.
if you still want discus, put six 8 inch diameter dinner plates in your tank, those are just about how big discus get, give or take, and think about if that would be fair for them. Discus should be kept in a group of 6, in a tank at least 4 feet long and 1 foot wide, preferably wider. If you keep them in a trio or smaller group like you wanted, there will not be enough of them to disperse aggression, and it may lead to casualties.