C.W.
Since it is a 3 yo's party I would just decorate with a complete mish-mash of things from her imagination. A little girl like that might enjoy a wall projector of the stars, if you used it to 'decorate' a wall or ceiling it could also be a present. I'd use lots of cellophane streamers and cool lighting effects and hang up a moon somewhere!
As to games I think her like for treasure would stand up to a simple scavenger hunt. You could have the guests make their own party hats from their imaginations (lots of construction paper and yarn). Playing dress-ups is a ton of fun at age 3, especially in a group. Good luck!