There is not a whole lot of difference in addition adults to young or young to adults. The same basic things come into play. A broody hen will often wean her chicks at 4 to 5 weeks of age, so it is possible for younger chickens to live with the flock. My broody raised chicks always live with the flock.
You’ve got a couple of different types of possible aggression to look at. One is pure integration. Chickens recognize who belongs to their flock and who doesn’t. Sometimes, not always but sometimes, some chickens will defend their territory and flock from outsiders. A lot of times this does not happen and in your case it is almost certain to not happen, but I’ll mention it anyway. This is where housing them side by side works to help. They get used to each other so they recognize their right to exist.
The pecking order is the other type of aggression you’ll almost certainly see and the reason in your case you probably won’t see the integration issue. A year old chicken is going to outrank a 3 month old chicken in the pecking order. She will dominate them. According to chicken society any lower ranked chicken that invades the personal space of a higher ranked chicken can be pecked to remind it of its correct place in chicken society. Once they settle in and everyone knows their correct place, this often does not happen. Once the pecking order is established, the flock usually lives in relative peace.
The possible problem with this age difference is that some chickens, especially hens, can be bullies. Some (not all) hens, if they clearly outrank the younger chickens, will go out of their way to be brutes. If a lower ranked member is pecked, it runs away and order is restored in the flock. But if the lower ranked member does not run away, this is considered a challenge to the pecking order and the higher ranked chicken can get furious and vicious. This is where space is important. The lower ranked chicken needs to have enough room to get away and to stay out of the higher ranked chicken’s immediate area. That’s why you often see two different flocks when you have this age difference. It also helps if they have different eating and watering locations to avoid possible areas of conflict.
With your adding one hen to three young pullets, it may work out fine. They are flock animals and the older one will almost certainly want to hang with the young ones. Whether or not she is a brute just depends on her personality. She will peck them some to establish her dominance, that is a given. But the key is whether she actively goes after them or not. The more space you have the better, but I’d give it a shot. You can always add additional perches and places for them to hide under or behind to create more separation. I think your odds of it working out are pretty good. But you are dealing with living animals. I can’t give you any guarantees.
Good luck and let us know how it turns out!
