I don't know how much room you have, that would affect my decision on how to proceed. From this and your other thread about purebred versus mixed roosters I think you have quite a bit of room so I'll go through how I do it. I'm not saying this is the best way, that every other way is wrong, anything like that. It's just how I do it.
I raise my chicks with the flock from day #1, either with broody hens or in my brooder in the coop. I usually integrate the brooder-raised chicks with the main flock at 5 weeks, though at times I move them to my grow-out coop at five weeks and integrate them with the flock at 8 weeks. Regardless they are across wire from the main flock at all times until I turn them loose. I have three separate coops/enclosures I can use to sleep them, a 12' x 32' main run, and about 45' x 65' in electric netting, so a fair amount of room.
I either hatch my own or get chicks mailed from a hatchery. I never add older chickens of any sort, so that is a difference with your three BR cockerels. I'm not sure how old they are, which again could make a difference. I normally raise 45 chicks a year in 3 or 4 different age groups. Some years I have a lot more cockerels than pullets, some years the other way around. That's my basic background.
Maybe 1 year in 5 it gets rough enough down there that I separate out cockerels from my main flock and raise them to butcher age in that grow-out coop/pen. The rest of the time they grow up with the flock. My laying/breeding flock consists of 1 mature rooster and 6 to 8 mature hens but I may have over 50 chickens of various ages down there. The ratios of pullets to cockerels doesn't seem to matter whether the cockerels get locked up or not. I go by observed behaviors more than magic ratios and each group has its own dynamics.
Most years I leave all the pullets and cockerels in with the adults. As they reach butcher age I eat the ones that don't meet my goals and give the others a chance to show me I should keep them. That usually narrows it down to 2 or 3 I have to choose between. Most of the time I make that decision when they are around 23 weeks but occasionally 2 or 3 get to grow a few weeks or even a couple of months older. Since I like playing with genetics I normally keep a replacement male every year but occasionally one stays around for two years. I normally train a dozen or so cockerels and/or cockerels and pullets to sleep in the grow-out coop, not for behaviors but to reduce the crowding in the main coop at night before I put some in the freezer.
I normally leave the mature rooster in with the flock until the new cockerel is about 7 months old. The older rooster tends to protect the mature hens from harassment by the cockerels, he doesn't do much to protect the pullets but he does keep things a bit more peaceful. Usually by 7 months the cockerel can take over the flock pretty peacefully but I once had one that took until 11 months to win over the last mature hen.
As they grow up the different aged broods tend to hang together and stay somewhat separated from the the other age groups, though there is often some intermingling. One of the problems with this is that the dominant rooster (or dominant cockerel if there is no dominant mature rooster) suppresses the behavior of the non-dominant males. I think the competition makes them more aggressive toward the hens/pullets than they would be if they were the only relatively mature male in the flock. Some of those behaviors can change as they mature too. Usually the cockerels mellow out as they become roosters but not always. Behaviors can change dramatically when the dominant male is removed. I can choose for physical traits fairly easily but the behavioral ones are much more of a challenge. I don't always get that part right.
As long as no bird is is being physically injured I tend to let them work things out with minimal interference from me. I can accept behaviors that would really freak some people out. But every 4 or 5 years even I feel a need to separate the cockerels.
If you were observing a feral flock, the normal behaviors is that the dominant male would drive the cockerels out of his flock when they reached a certain point of maturity. They would form a bachelor flock until an individual matured enough to start carving out his own territory and start attracting hens and pullets to form his own flock. Or one might challenge the dominant rooster in an attempt to take over his flock. That doesn't suit my goals and I don't have that kind of room so I just do the best I can. Removing the cockerels from the flock and into a bachelor pen probably mimics the natural behaviors closest but how do you judge behaviors, imperfect as that judgment may be?
I don't know what the other cockerels you have to choose from look like. With your BR pullets you are likely to get black sex links if the male is not barred. The males will be bared, the females will not. It depends on what genetics that male has but the first generation from your BR pullets will likely produce black barred males and solid black females. The black and barred genetics are that strong but as always there can be exceptions. It's the second generation where you start to see different patterns and colors show up, especially with a non-black rooster. I don't know if that will influence our decision or not.
I'll emphasize that I very much believe how much room you have will have a huge effect on their behaviors. The more room the better. Hopefully you will get something out of all this to help you in your decision but it is usually not that easy a decision to make.
So much for having an extra! He was very nice, but that sort of thing can happen too.
