There is always a chance that certain pigs will not get along. Really, this depends on nothing more than personality, not really numbers or sex. If it is a very reputable rescue that has already done a 3 week quarentine with the potential adoptees you may look into bringing your pig to meet the pair. I've had success with this in the past. If this isn't an option then the best thing to do is to do everything you can to make it easier for them to work out their dominance issues.
I use
this thread on guinea lynx as a guide for introductions. There's a lot of good tips and some pictures and videos of normal behavior for introductions.
Questions you need to ask yourself:
-What's the biggest I can make my cage?
A 2x5 is recommended for 3 boars, but the biggest cage you can make is best.
-Do I have room for two cages in case I end up with incapatable pigs? Is the rescue willing to work with me and switch out pigs?
There's always a chance that some pigs just won't get along. If there's continually failed introductions that lead to bloodshed, you need to have a plan of action.
-Am I really up to taking care of three pigs?
This seems obvoius, but really be sure you're ready for that number.