adding another one should be fine. just add them to the new tank at the same time before the 2 that are use to each other get a chance to establish territories. if that's not possible, no big deal. just feed the 2, rearrange the decor in the tank, add the new one, chase all 3 around with a net to scare the crap out of all of them and after they run around they'll all 3 be best buddies.