You have 2 boys? Are you meaning to put the new one with them? If so I would not advise it, as from previous posts it looks like they still haven't established dominance, they still have there teenage stage to get though! If you put another boy pig in that could upset things further!
Also you would need a bigger cage, which can be expensive and if they fall out you would need lots more cages!
You 100% can't put a girl with 2 boys they would fight over the girl(not to mention if she wasn't spay she could get pregnant), Trios in males are extremely rare and even the most experienced piggie person would struggle with a trio but can be done but I wouldn't advise it, IT's all down to the pigs and there personalities. Theyare not very common!