One of my boys settled and was happy to be around me after six weeks, he was taking food from me and came out to see me. It took the other one a year and a half to stop running away from me. They’ve been with me for three years and now both take food from my hand, and are happy to potter around me But they both do not like being handled and held.
Its nice you want to keep them together, it does feel sad to separate them but that is from a human point of view. Actually, separating them, particularly now one has sustained an injury, would be best. Boars very rarely function as a trio. Separating them into a pair and a single would be best now sadly.
While dominance behaviours such as rumbling are normal, when things turn to the extent that an injury occurs, then it is the end for them. Any fights which cause injuries or blood mean they do not like each other And do not have a functioning hierarchy.
A Pets at home cage isn’t likely to be big enough, not even for two let alone a teenage boar trio. Two boars need a cage measuring 180cm x 60cm. Do give the cage a measure and check it.
Three boars would need a cage measuring 300cm x 100cm (three square metres) but space alone will not fix a dysfunctional relationship or heal rifts. So even if you get a bigger cage now, they likely will not be able to remain together now injuries have occurred.
Commercial cages are rarely big enough for boars. The biggest commercial cage is the ferplast plaza 160 which measures 160x60cm so can still be a little on the tight side for two boars (particularly as teenagers). A c&c cage is a good option for boars as they can be made bigger to be more suitable (a 5 grid by 2 grid cage is recommended for two boars)
Please do not add a fourth In with the three of them. Adding a fourth is even worse than three. You can cause even more major issues by doing so and you could end up with four single piggies (all needing their own cage and their own new friend) if you were to try it.
As I said, given injuries have occurred and this doesn’t sound like a harmonious living arrangement (a functioning pair with a good relationship don’t try to stay out of each other’s way), then the best thing would be to split them into a pair and a single - choosing fhe two piggies who get on best to remain together. The single piggy will need to be kept alongside the pair for interaction through the bars only to stop loneliness. Then work on getting two 180x60cm cages. It would then be best if you could then take the piggy who ends up single, boar dating at a rescue centre to find him a new friend. That way he can choose his own friend and you can be assured of compatibility. The rescue can carry out the bonding process for you. You can then keep two separate pairs. This will work out much better than trying to keep a trio. (With the pandemic, dating has been suspended in some rescues, so you would need to check what options you have with a rescue centre - provided the single is kept alongside the pair, then he won’t be lonely so you would have time to get things sorted for him)
Buying piggies on spec can mean you run the risk they won’t be compatible and therefore wont be able to live together. This is even more of a risk given your piggies are hormonal teenagers. This is also why getting the help of a rescue centre is valuable. Bonding teen boars can be harder.
Once the single piggy has a live in friend of his own, then he no longer needs to be kept alongside the others so to save space if necessary, then you can stack cages (either one on the floor and one on a table; or if you do use a C&c cage then they can be stacked)
Do read the green links I added in above (and below) as they explain things more fully.
Cage Size Guide
Member Gallery: C&C cages/homemade cages
Dominance Behaviours In Guinea Pigs
I couldn't agree more with the excellent advice you have had above.
. Hope everything settles down once they are separated.