It’s done via neutral territory introduction on an all or nothing basis. You put them into a bonding pen and leave them in there for a few hours to see if they like each other and can form a rough hierarchy. It they can’t, then there will be problems in the bonding pen and the bonding will fail.
If they do like each other, then you clean out the cage they are to live in so it is also neutral, then move all piggies to the new cage. They then go through a period of two weeks while they fully establish their bond and relationship. Things are usually ok by that point, But it can still suddenly fail within those two weeks also.
You must never just put unknown piggies into the cage of another piggy as it’ll be seen as a territory invasion.
You should also quarantine any new piggies if they do not come from a reputable rescue centre so the risk of passing on any illness is removed. As your piggy is only 8 weeks old, if the other two boars have gone to new homes by the time you bring any new (older piggies home), then you will not be able to quarantine the newcomers as your piggy is too young to be alone at all. You therefore do need to take the risk that they may in fact bring something in and Then you could need to treat them all.
If you still have all of your baby boars when you bring any newcomers home so the baby isn’t alone, then the older, new piggies, would ideally be quarantined in a separate room in separate cages from each other for two weeks. You can them See which one of your baby boars gets on with the sows and rehome the others.
Doing any bonding yourself (not via a service at a rescue centre) with piggies you do not know, is a risk. As he is young, the sows may accept him easily, but once he hits his teens, things may change. The sows may also not accept each other or they may not accept him from the beginning. You would need to plan three separate 120x60cm cages in case it doesn’t work out.
This are the bonding guides. Please read them all before you attempt to do a bonding yourself
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?
A Closer Look At Pairs (Boars - Sows - Mixed)
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
A Comprehensive Guide to Guinea Pig Boars
Dominance Behaviours In Guinea Pigs
Bonds In Trouble
Importance Of Quarantine
Sows: Behaviour and female health problems (including ovarian cysts)