Yes and no
Trios and quads work best when all the piggies are all adults, as if you introduce a baby they'll more than likely fall out during the hormonal months.
Trios work on all the boars being laid back, or two laid back boys and one dominant, so the right personality is key. As well as laid back personalities, trios and quads need ALOT of space. It will not work with the three boars being kept in a small cage or hutch.
As well as a lot of space; time, money and consideration must be thoroughly thought through before introducing another boar if your two boars are laid back. These questions must be thought through before thinking of adding a third boar:
1) What happens if all three fall out? - Do you have spare hutches/ cages for the three boars , what will you do with them if they're not best buds will you put them with sows?Do you have enought pennies for this?
2) Do you have the time to introduce the third boar, patience is needed and introductions must be carefully planned.
3) Space, what cage/area/hutch/shed are you planning to house them in?
Here is some good threads/sites:
http://theguineapigforum.co.uk/showthread.php?t=67202&highlight=3+boars
http://www.susieandpigs.pwp.blueyonder.co.uk/optionsmenu.htm
I'd strongly recommend contacting owners with trios/quads, rescues etc who have had experience if you are thinking of going ahead.
Ooo and another neautering wont change their behaviour