First, I’m sorry for your loss.
Adding a third boar to a bonded pair will almost always upset and with the potential to destroy the bond between the original two. Unfortunately keeping three boars together (even if they are siblings and/or were always together as babies - as soon as the teens hit they usually fail and most boar trios will not make it to adulthood together) almost always results in trouble, fights and broken bonds. Boars can only really be kept in pairs to be successful. Most people cannot provide enough space for a trio - they need a square metre each and there are no commercial hutches which cover three square metres so they arent ever big enough for a trio. A boar pair need a 6ft x 2ft hutch/cage
When bonding you cannot do little meetings here and there, it has to be all in one go, success or failure in a neutral territory bonding pen and then permanently living together from then on and not separated again (if successful) or permanently apart (if failure). Having a piggy living alongside is not bonding at all, and little meetings here and there is also not bonding as they start the process but then it gets stopped. It causes stress and means that every single time they have to start over again. Babies (under four months of age) need immediate bonding and they shouldn’t be alone in a separate cage. As a bond comes down to character compatibility and mutual liking, buying a piggy on spec means you have no idea whether ther is going to be character compatibility or mutual liking so there is always the risk of failure, even more so when bonding a third boar in. However, due to the fact three boars almost never works out, trying your pair with the new piggy has caused problems in their relationship. They may or may not be able to get it past it and remain together but if they are really full on fighting, then it’s not likely to be good news for their relationship.
The baby is far too young to be by living by himself but you won’t be able to bond him in with your current pair without really causing problems either now or down the line even if your current pair werent already having problems.
You need to follow the bonds in trouble guide to work out whether your pair can have a functioning relationship and whether this has brought up issues between them. They may need to be separated for a couple of days to calm down and then tried together again on neutral territory to see their reaction - if they accept each other back, then they can likely stay together but please don’t add any more piggies to a boar pair; if they immediately fight (not dominance behaviours, but fighting), then it’s over for them . If they really are full on fighting now, then it’s immediate and permament separation without trying a trial separation and reintroduction.
if your pair can work things out then they can stay together and you will need a new friend for the baby.
if your pair can’t work it out and are full on fighting, they will not be able to remain together. You can then try bonding one of them with the baby. But you cannot do it big by bit. You have to put them together in a neutral area and see it through. You will then need a new friend for the single piggy.
I’ll add in some guides Below
Bonds In Trouble
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
A Comprehensive Guide to Guinea Pig Boars