Labrador retrievers are notorious for being reckless eaters, one of the main reasons for their weight gain. If your 3 month old Labrador Retriever puppy is munching on anything available, we would suggest puppy proofing the house so that he does not eat anything in sight. Also, do not feed any table scraps to your pup. Maintain a strict feeding schedule and ensure ample amount of exercise for your pup.
Hope this helps!