I don't think rescuing another guinea pig is ever a bad idea unless you are concerned about your own resources and being able to properly care for him.
And while it may not be a bad idea, there are many things you need to consider and plan for: like what you would do if the new boar does not get along with the two you already have? Or what you would do if it changes the relationship of the two you have and they also start to not get along? What methods would you plan to use of introduction? How long are you willing to try introducing them?
Will you need to expand your C&C, buy/make new hay racks, change your feeding schedule/arrangement, or serve different veggies than what you are already feeding?
As long as you have the means, resources, & love to care for another pig I can't imagine rescuing another would ever being a bad thing. Just make sure you cover all your bases just in case it changes the dynamics of how your duo is currently living.