Zebra finch bullying......................…

I recently acquired 3 male zebra finches. they were rescued from a neglectful owner. I already have 2 budgies so I agreed to take in the finches. I moved all 3 finches to a temporary cage and cleaned their cage and toys and transferred them back. One of the finches is being ganged up on by the other 2. they pick on him constantly and I'm not sure how to stop it. Should I remove the one being bullied to it's own cage or buy another finch to pair with the "outsider"? Any advice?