Hi and welcome to EBN. Bully is super cute. I don't have any experience with arthritis in dogs, and we don't use Chrondroitin with our two guys, but I would agree with your vet in that I think Bully may be to young at only 3 months old to be worried about arthritis yet. As long as he's getting a good quality puppy food, he should be getting enough nutrients to supply him with proper growth and development. When he gets older, about 6 or 7 months old, you can switch him to a good quality adult dog food and add some natural supplements. We give our guys 2Tbsp. Of plain unsweetened yogurt to their morning kibble, and we give them 1 Tbsp. of salmon oil in their evening kibble, plus they get 1 Tbsp. of apple cider vinegar in their drinking water everyday. If when he's older you want to revisit adding Chrondroitin or other supplements, you can consult with your vet and do your research as to what supplements are good or needed. I don't mind giving my guys the supplements I do, because they are all natural and healthy. and so not dangerous.