I would suggest Kobata's DVDs for footwork. Specifically off foot throws, cuts, and bunt coverage defense (footwork).
You can drills from now until the end of time and still not be a better defensive 3B. If this were true I could make anyone a great defensive player by simply doing drills (investing time).
You need foundation. There are others besides Kobata, but I like most of his stuff.
Seconded.
1. Glove work. 2. Footwork/throwing angles (agree on Kobata). 3. Positional knowledge - ground ball fielding "territory"; pop up territory; BUNT ownership. Pickoffs and steals.