Toys, toys and more toys!! And catnip too. And lots of play time!!!
My first cat, Russell was an outdoor cat and he is now an indoor cat. Though I did get him as a kitten but I don't think it was any easier. He used to bolt out the door as soon as it opened even when it was raining.
But with lots of play time and plenty of toys around, scratching mats and cuddles and time for himself to do catty thing alone, things settled down six months later. I'm not saying that it will take that long, it just took that long in my case.
Though, you do have it slightly harder with two resident cats already. All three cats will have to get used to each other and establish a pecking order of their own. Feliway does have a calming effect on cats, though it won't curb their desires such as wanting to venture outside.
Encourage your new cat to play with cat toys inside. I have found that baby toys last the longest with my two as they are less likely to fall apart and less likely to become lodged in their throats and become choking hazards. But there are good quality cat toys around if you look hard enough. And have plenty of toys especially if the resident cats play with toys too. (That in itself could get interest from your new cat to play as well.)
And maybe purchase a new scratching mat/post that has no one's scent on it so everyone has equal opportunity to scratch it and mark their scents on it. The new cat may see the original scratch post as claimed territory and not want to scratch it leaving him nowhere to scratch other than perhaps your favourite couch!
And perhaps make a toy to allow him to use those hunting skills so he can hunt inside. I tie a piece of string to a favourite toy and pull it so it trails in front or either of my cats. And it nearly always gets their attention and they begin stalking it before pouncing on it.
And eventually, he'll realise, there's food inside, somewhere to scratch, somewhere to sun himself and sleep the day away, things to play with, things to hunt, other cats to play cat chasey with, somewhere to go do his 'business'and free and loving cuddles and the outside will seem less attractive to him. Though he may retain some of his desire to venture out. You can help alleviate this by making a place for him to look outside at everything that wanders past a window or maybe take him for walks with a cat harness.