The only way you can know is to try it. Whenever you put a group of fish together some kind of heirarchy is worked out, just how aggressive it becomes depends on the species and individual fish. Just because all three of these fish were being picked on in the main tank doesn't mean that one of them won't become overly dominant in the 20 gallon.
Something that might help is to place a large fake plant in the 20 gallon. It will help to provide a visual barrier between the fish and also help to prevent any of them from getting a clean shot at the others.
Good luck with the healing process.