Complex sentence: By Season 3, he has been with Don three years, and there is nothing Don cannot ask David to do, though, Don worries David may one day realize he has seen too much and leave the FBI.
Split into simpler sentences: By Season 3, he has been with Don three years, and there's nothing Don can't ask David to do. Though, Don worries David may one day realize he's seen too much and leave the FBI.