I really don't want to sound mean, but yes you would be technically "cheating the system." My dd is 10 now and I hate the fact that I have to pay an adult price for her, especially for the dining plan because she eats like a bird, but rules are rules and policy is policy. Disney has to draw the line somewhere and if she is indeed 3 when you go then you should pay. It really is no different than me having to pay an adult price for a 10 year old that is by no means an adult. But, ultimately it is your decision. Whatever you do, have a great trip!