Feliway is great stuff! Also I would check to make sure he is ok in himself at the vet's. It could be a sign that he isn't feeling well. My former flatmates neutered male cat used to spray when he wasn't feeling well.

It could be behavioural with the *territory* being invaded by newcomers but that should only last a couple of weeks. Moving home can be really stressful for any animal so he might need some kitty prozac just to settle his nerves for a little while. I put bastet on that for a couple of weeks because she was spraying at my new place of residence and it solved the problem.

Also do you spend time with Harper first? Then the other 2 newcomers? If you don't maybe he is trying to tell you that he is not happy with the situation. Try to spend some quality Harper time and see if that helps.

I think its great that you rescue cats. People like yourself and many other here are a real blessing to the animal community.