I'm going to eat at a buffet
Are you that hungry to eat at a buffet?
I just want to stuff myself full of food
It's fine to indulge once in a while, and buffets are the perfect place to do so.  However, you shouldn't be a glutton.  Don't stuff yourself too often, because this will be bad for your health.
I only want to eat three servings. What's the harm in that?
It depends on what kind food we are talking about. Three serving is a bit too much for any plate.