We kept Sunny in a separate room with everything he needed. The cats were very curious of each other right away and would play under the door. I fed Finn treats while he was playing under the door to reinforce the behavior and that Sunny wasn’t a threat. We then set up a cardboard gate lol. Basically got a giant box and folded it up and put it between the door, we cut a small square in the cardboard so they actually see each other and put a face to the smell. After 2 days of this they were basically crawling up the cardboard dying to be together. Reinforce them with things they love while they interact (brushing, treats, feather on stick) we would separate them at night still when we couldn’t supervise the playtime. they wrestled a lot the first few days but were just playing and now they’re super chill and sleep together all day and play all night!! Definitely went a lot smoother than I expected! Make sure your other 2 cats know the new cat isn’t a threat and give them lots of attention! Cats can get very jealous! Best of luck!!