You were meant to find this dog. He's going to need someone who's understanding and has the patience to deal with his shyness. People looking to adopt from that shelter aren't likely to want a grown dog with issues and he'll get passed up because he's not perfect. If you're ready for the commitment of 3 dogs, go for it. You know he gets along with your dogs and that's the most important thing, they will be good influences on him and help bring him out of his shell. I agree with Karen, get him a crate or a room he can call his own. Once he's had a taste of the inside life, he won't want to go back out.