So when I use Donphans Rolling spin attack it says During your next turn this pokemons rolling spin attack does 70 more damage. So if I use if for 3 turns does it do 210?
No. Card effects don't stack in that manner, unless they explicitly say they do. "70 more" means more than the base damage, so it does 140 every turn after the first (so long as it keeps using the attack and doesn't hit the bench).
That does not stack either, so it does 60 the first turn, then 120 every turn after that until Metagross hits the bench or is KO'ed (or doesn't attack).