Should you not let your puppy play too mch if he has a grade 3 heart mermer?
On the contrary, exercise and activity are good for murmur dogs. They leaner and more fit you keep them, the less strain on their heart. As long as he is not wheezing or lethargic, I wouldn't worry. Sometimes puppies can even grow out of heart murmurs, or so I've heard. I'd like to add, make sure you take care of his teeth, as there is a connection yucky teeth and heart problems.