I have 3 red bellied piranhas that grew up together one was injured? do i put it back in the tank?

Hello I have 3 red bellied piranhas in a 55 gallon. Frst I know that the tank is a little small so I am upgrading it to a 75 gallon soon. But in the meen time I had the piranhas in the tank for a year and a half. They were doing fine until recently the smaller piranha (is a little smaller than the other two but you wouldn't really notice unless u actually pointed it out) git stuck under a log I had in the tank and his tail was bitten at( because he was stuck) pretty bad and his one eye was injured very bad and seemed as if it was going to fall off. I took him out and put him in my 30 gallon hospital tank and am currently using melafix I believe that's what it is called to treat his wounds his eye looks A LOT better but I don't think he can see from that one eye and his tail is still pretty bad but has healed a little and I am planning on keeping him in the hospital tank until he is completely healed. After he is healed I want to put him back in again with the other two and I removed the log. How should I put him back in? I thought of an idea that might work its to separate the other two a couple days prior to puting the other piranha back In with a tank divider so they arnt used to eachother when I put my other piranha in the tank I will take out the separator and let them reunite so not one piranha will be the "new" piranha so the other two won't gang up on the one piranha. Any other ideas or help? Do u think my idea will work? Please help me out on this thanku by the way the piranhas are about 5 inches and like I said before I will upgrade when I got the money to. Thanku for your help any help is appreciated!