If I could wind the clock back I wouldn't get our third cat again. Although he is lovely, my other two are sisters and six months in its no better. They hate him, he tries to be friends and does all the submissive stuff but likes to run around and chase them so he gets hissed and growled at from the top of my wardrobe.
We've tried scent swapping, slow introductions and feliway but none of it has worked.
I'm not trying to put you off at all as two cats who get on are an absolute pleasure, just letting you know it doesn't always go smoothly.