Welcome to the forum.
As you have now found, sadly, three boars together almost never works out. Getting the character matches right is incredibly difficult in a boar trio and almost always these kinds of fights occur. Most boar trios will fail when hormones start to kick in from 16 weeks with around six months of age being a danger time but the majority will never make it to adulthood together (14 months of age). These kinds of fights occur because they did not in fact like each other enough and were not able to form a proper functioning hierarchy (its because two of them want to be dominant, one will not back down and then things deteriorate rapidly). In a functioning boar bond you have two piggies, one dominant and one submissive and this is why a pair of boars works best. Space is also a major consideration when attempting a boar trio - the space needs are huge (in excess of a 3 square metre cage is needed to attempt it - floor time is fine but if the cage they are locked into is too small then it will escalate tensions) but even with lots of space, because character comes first, things still will go wrong.
Now this has happened, they do, sadly, need to be permanently separated. Their bond is broken and they cannot be together any longer.
If Nibbles is still ok with them both, work out which one he definitely gets on best with and ensure they can remain together as a bonded pair. The cage size requirements for a boar pair are a minimum of 150x60cm cage but 180x60cm is best. The piggy who is single will need to be kept in a separate cage alongside the pair.
You cannot keep the single piggy in the upper level of a two tier cage as unfortunately he will not be able to have full interaction with the pair and will therefore likely become lonely. So, he will need to be in a minimum of a 120x60cm cage side by side with the pair to enable full through the bar interaction.
Unfortunately because bonding teenage boars is difficult, you may now find it harder to find him his own new friend. Living alongside the others will stop loneliness and it is a suitable option. However, given his age, it could be best if he could have a new live in friend for him to spend the rest of his life with.
This is where things can become tricky though. Bonding via dating at a rescue centre is best as he can choose his own new friend but given the hormones are now in play, bonding teen boars is difficult some rescue centres will not bond teens due to the issues they face. You can enquire of them though and see how things go.
The other option is having him neutered, having his six week wait to become infertile and then bonding him with a sow. The issue here is that if he was to live with a sow, then he and his sow wife would need to be kept well away - ideally in a separate room - from the boar pair. For the boar pair, smelling a sow could also break their bond and cause them to fight.
Anyway, for now sorting out living arrangements so they can all be kept on the same level for full interaction through the bars only and ensuring the boar pairs cage is big enough (6ft x 2ft) on a single level is the important bit (two tier cages are fine but only if the bottom level meets size requirements as upper levels dont count towards the cage size. Upper levels are bonus spaces only). You can think through the options going forward once the shock of their fight has worn off and you've had time to digest things.
I've added in some guides below which will help.
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
A Comprehensive Guide to Guinea Pig Boars
