The legendary Dale Earnhardt made the number 3 famous in NASCAR. When Earnhardt died in 2001, the 3 basically died with him. But it was never retired, and next year it returns to the Sprint Cup Series with Austin Dillon at the wheel.
Austin Dillon is the grandson of Sprint Cup team owner Richard Childress, and has used the 3 since he started racing. It was a foregone conclusion that once he moved up to NASCAR's premiere series, he'd use the number 3.