For example, say a Pokemon was Grass, Flying, and Ground and is hit by an Ice Beam. Would the Ground be cancelled out by Grass and it just deal x.5 damage? .
This one is easy, considering Grass/Flying/Ground has 3 weaknesses to Ice Beam. 2x2x2 = 8x damage.
I think what you MEANT to suggest was Grass/Flying/Water, where there are two resists and one weakness. I assume it would be 2x2x.5 = 2x damage.
8x weaknesses and 8x resistances is probably too powerful. As an example, I know that my Vileplume is going to die to Blaziken, but it's not like Leaf Storm is completely ineffective, it will still do ~30% damage before I die.
