Hello and welcome to the forum

Are your other two males already bonded? If so, it will be unlikely that they will accept another.
If you want to try, you have to do it on neutral territory, somewhere none of them have been before like the bathroom, hall or even the couch! or set up a run or pen. Beforehand, you could hold your lone guinea pig and just let the other two sniff him or have them next to eachother with a divider between them and see what happens. If that goes ok after a few times, try them together in a neutral space with clean towels or something similar.
If they are going to fight, it will start with teeth chattering and they will square up with reared heads. If this happens, have a towel handy to hrow on top of them and separate. Dont get your hands in the middle of it otherwise ouch!
If it were me though and the other two are already bonded, I would consider getting another boar to bond with the lone one. There are still no guarantees but may be more likely to work.
There are a couple of members on here with trios of boars, so hopefully they will be along to help you. I have a bonded pair and a pair that fell out so they live next to eachother now
