I disagree that they automatically become aggressive as they mature. As long as their needs are met, they will stay fairly peaceful and leave your other fish alone. They do prefer a meatier diet as they get older, so feed them a sinking tablet designed for omnivores or carnivores. I use the one designed for cory cats. They will also need at least two hidey holes per
CAE, so with three of them, I would suggest at least 6-7 caves or hollow logs or ledges that they can hide in or under. They will each eat at least 2 of the pellets each night when they get older. They also like the shrimp pellets although not as much as the tablets. I have not had any problem with mine. I had three in my pond all last summer and brought them in last winter (it was too cold outside for them). This spring my gold - color changing one went back in my pond and the two normal colored ones went to my sister's pond (700 gallons and filled with algae). Her algae has been cut down by over half since the two cae's went over this spring.
Basically, as long as they have enough room to claim as their own and are fed enough, they will leave your other fish alone.