If you just got him three days ago there's a good chance he's still nervous about his new surroundings. A kitten than young needs to feel super safe and secure - which means having people around who are calm and loving and make him feel safe and secure.
A change in his diet could cause digestive problems.
Only a vet could tell you if you have reason for concern. If there's a way to ask whoever it was you got him from whether he had the problem before that might help you figure out if this is something new for him. Cats that I've known often eat a lot when something has gone on that has made them nervous. Eating too much can cause a cat to have digestive problems. Based on cats I've known.. I wouldn't give a cat any of the foods/treats out there that have "greens for cats" in them. Every cat I've ever run into who has been given "greens-for-cats" food/treats tends to get digestive "issues" from eating that stuff. I only mention that because someone not too familiar with cats may think the "greens-for-cats" kind of food/treat seems like a healthy idea; so I thought maybe that could be ONE possible cause of the diarrhea "issue".