First of all if you are going to do a triathlon you need to eat exactly the opposite of an off season bodybuilder. You can't be shredded and gain insane amounts of mass at the same time, pick one or the other. I've seen it happen so many times, skinny teens come up to me and ask what does it take to get big and when i tell them, they are allways shocked and when i see them a year later they look exactly the same. Skinny, week and sad...
LOTS OF RED MEAT, CHICKEN, FISH, EGGS, DAIRY, RICE, POTATOES.
HEAVY FREE WEIGHTS AND COMPOUND MOVEMENTS ARE THE KEY.