He just wants attention. Some dogs are naturally chill or naturally independent and will relax around the house while you do your thing. Others are not. I have a pup (well, now an adolescent) who naturally has two settings: sound asleep, or cranked up to 11. "Tire them out" is usually good advice, and it worked with all my previous dogs, but I discovered with this pup that he, when tired, would actually get MORE amped up, like an overstimulated toddler. Obviously this made him quite the handful around the house, since he was hyper when not tired, and hyper and crazy when tired. (But pop him in a crate while in that state, and he's sound asleep in minutes.) I have learned the hard way with him that some dogs are just...extra. My hyperactive puppy would play with the other dog or by himself when I was up and physically occupied with something (e.g. washing dishes) but if I sat down for a minute, he'd be all over me, which made it impossible to do stuff like read or go on the computer without going in a different room or crating him, and obviously I don't want to crate him every time I'm going read/craft/game/go online/watch tv/etc.
Search for "sit on the dog" for an exercise in teaching a dog to settle around people. Given that your pup is only 3 mo old you may need to shorten the recommended times quite a bit. This method was a lifesaver for me. I started off feeling grateful if I got just a few minutes at a time of calm at time with my puppy, and am now up to him being able to settle next to me for about 45 minutes, long enough to read a few chapters or watch an episode of a show. I think it will get easier as he gets older, too.
