Introducing a third rabbit to an existing pair always runs the risk of breaking the established bond. It's practically impossible to have a trio with more than one male. Some have had success bonding trios of all females or 2 females and 1 male.
The general thought is that trios are very difficult to achieve and (as already stated) run the risk of breaking the existing bond. I can't speak for how to go about attempting it since I have not had success with a trio (though I've tried). Any bonding info I've read said it can't be done. But I know there are one or two RO members who have bonded multiples.