Summary: The Third Doctor makes the best of his exile on earth.
This isn't permanent; he's not putting down roots or giving up travelling. He'll be back out there in no time, or sometime, because time's one thing someone with a TARDIS has an almost unlimited supply of. His travels are simply on hiatus, and why not? There's no rule that says he can't stop for a while.
Besides, there's plenty going on here to keep him occupied for years, if he chooses to stay that long. Assisting UNIT is interesting enough and he's met some fascinating people.
Adjusting his cape, the Doctor settles into the driver's seat; the universe can wait.