While having a lot of room would be to your benefit, the "need" is that they have to decide which one is dominant. That could mean a fight to the death or, like your other two, they could reach some accommodation. Often the boys establish their own territory and attract which girls they can but sometimes they find other ways. And sometimes chickens die or are seriously injured. A lot depends on the individual personalities of the boys.
If your Silkies aren't attracted to the other two roosters why do you think they'd be attracted to a third? I personally don't consider a rooster with the flock to be that much of a protection against predators. I've lost chickens to dogs, foxes, owls, and hawks. The dominant rooster did not stop any of these predators and was not injured. Mine try to lead the flock to safety instead of fighting a rearguard action. If something is suspicious a rooster will often get between whatever that is and the hens to check it out so they can be picked off. It's not that they are fighting the predator so much as they get just are a little more exposed. Most critters that get chickens are ambush predators so the hen is gone before the rooster knows hat is going on. I personally don't count on a rooster to keep them safe.
The AC trio are probably too old to be raised as members of the flock but are still immature juveniles. You can still integrate them but especially with the boy it becomes a little harder. Still people integrate them all the time. Sometimes it works, sometimes not.
You can always try. It might work out, it might not. I generally suggest you go by what you see rather than what some stranger over the internet like me tells you that you will see. Free ranging gives you a lot better chance than people that have limited space. Having a different place to have for them to sleep separately is a benefit but some times they can share sleeping space. You never know how it work will work out when you deal with living animals, anything can happen. When you free range your chickens are at risk to predators, with or without a rooster. When you try to integrate anything can happen. It may be an unbelievable success or abject failure.
