Fantasic success ruling

so with the power sniping, it says x3 damage, say I roll a fantastic and do 84 damage (with the fantastic double), would I only add another 42( damage without the fantastic doubling it) or multiply the 84 by three?

You would triple the base damage of 42. You would not triple the double; that would give you x6 damage. The default double damage on a fantastic success only applies for basic attacks with melee punches and melee/ranged attacks with weapons. When you use a power, it will tell you how much extra damage you do with the power if you do any extra at all. There might be some exceptions, as there are several editing issues with powers.