a 55 gallon is too small for 3 RBP for life, u could probly keep them in there till they reach5 inches each, but any bigger and your gonna have stressed fighting P's, i evean think a 75 gallon isnt big eneough for 3 RBP, i would say a 100 gallon would be ideal, i have 6 RBP in my 100 gallon,all at about 5 to 6 inches, another couple months and 3 will be headed upstaries to my other 100 gallon, they willl reach there full potinatle like this and hopefull cut out terrtoal fighting, heres a pic