No way would I put 3 of these in a 55. They get fairly large and tend to school. They wont have enough room in a 55 even if there were no other fish or rocks. I have 3 in my 300 gallon with 2 eels, a puffer, and a clown trigger. They are about 8" each, and very active swimmers. I wouldnt put a school of 3 in anything less than a 150.