I am no expert, but I rehomed 3 individual lady buns from a neighbour who was struggling with looking after them and her young family, and have spent the last year successfully bonding them. it is a long , slow process . Two of them are related ( mother and daughter) and then there is a baby ...unrelated...but all three had lived apart. I started by spaying all three as females are so aggressive entire. I still kept them separate, but let them live in conjoining cages, where two sides of each cage faced into the other two buns, so each bun had another bun on each side. There was scrabbling and fighting through the bars to begin with, but this eventually settled to pooping and glaring....I let them live like this for about 4 months. I then started swapping hay and litter trays so they got more used to each others smell...for about 2 months.... I then swapped each bun in to each other's cages, so they were immersed in two other buns smell and bed etc. Then I put them all together one day in a small carrier and drove them around...some may not approve of this forced bonding, but it really worked...I kept them overnight in a small space so they could not scrap and then slowly enlarged the space and they all live very happily together now. It was a YEAR long process...and I really had to stick at it and not loose heart.
I would really take the all advice from far more experienced owners above and let your baby boy grow a bit stronger and heal from the neutering, and , if you really do want them to all live together, perhaps start by separating your already bonded pair. Be observant at all times, I wish you the best of luck as it is a tricky business.