My advice about adding to a pair of boars -DON'T
More often than not having more than a pair of boars leads to everyone falling out and that will leave you with single piggies living seperatley.
Also your boys are still young and any day now they will be both going through the stroppy hormonal stage and 2 piggies sorting out the hierarchy is enough to contend with at the minute.
There is an exception to the rule and some people have successfully kept 3 boars together, sometimes they seem to work at first but a lot of the time they have fallen out.
The key element for boars is lots of space, they can bicker over space, houses, food bowl etc.
A massive factor is personality, if one piggie is laid back and it isn't in his nature to want to be top boar, the other piggie can take top postion easily. Its when 2 boars are quite dominate and they both want top postion that squabbles, and even fighting starts if they cant sort it out.
Also with a trio I have often heard of one getting bullied to the extent he wasnt allowed to eat.
Good rescue places will bond boars so they are matched on personnality and not just pot luck that they like each other. We call this 'Boar Dating' and it is very successful in finding boars 'friends' not just cage mates that tollerate each other.
I rehome boars from a rescue I foster for and I recommend at least a 5ft by 2ft cage minimum for 2 boars, although my own boars are housed in much bigger C&C type cages.
Here is a brilliant site about BOARS
http://www.susieandpigs.pwp.blueyonder.co.uk/optionsmenu.htm