If he hasn't moved an inch in three weeks then it might be a good idea to try and poke him to see if you get any kind of reaction, even in deep premolt it's unusual for a tarantula not to move at all, I'm going to assume you bought this tarantula from a pet shop?? If so judging by how fast it burrows it's probably one of our obligate burrowing arid species, what's his legspan?