I’m not the best one to answer here but if I was in your situation ( older one stressed by the younger one and it is effecting his condition ) I would seperate the two now.
Then focus on bonding the younger and the new one after a couple of weeks break ( cage them near each other but not close enough to nip through the bars in the meantime)
If you bond the younger two with the older one it will add more stress to older one for nothing as he isn’t staying with them. Also adding a third can effect the bond between the existing bonded two so may result in fights especially if older one is already stressed
I would say if the old one is truely bonded to the middle one you will have to go slow – cage all three seperately with the middle one in the middle of the one he was bonded to and one you intend to bond to.
Slowly make the gap between the old one and middle ones cages bigger will leaving the middle and younger one still caged close to each other.
If older isn’t stressed by this and his condition is improving as you previously saw when separated then you know he will be fine being separated to other side of room from middle bun.
Before you look at bonding the middle to younger make sure the seperation between middle and older is complete.
Also that the two you want to bond have been caged near each other for a good wee while first AND the newest needs neutered for 6weeks prior to bonding starting.