EDIT: Missed the part where you said you already saw the first movie. Anyway, like Waverer said the new movie is only taking the basic premise of the third game and making a new story around it. So playing SH3 before it won't spoil anything. You may as well whenever you have a chance to play it. It's a good game.
This movie is to SH3 as the first movie was to SH1.. meaning it will be a new take on the same basic idea. I don't reckon it will make any difference but personally I would play SH3 first, then see the film... however, that's coming from someone who already knows what the game is like as opposed to what might happen in the movie.
Uh sure play the game, though I HIGHLY doubt this movie will take more than a few scenes/references/monsters from the game... just like what they did with the first movie. Still play SH3, its an awesome game