No such thing as a "Class III item." Whatever those are. Weapons are either Title I or Title II. The Class refers to the type of FFL. Since most people buy NFA weapons from a Class 3 dealer, this misnomer has stuck (to my annoyance--pet peeve).
Your friendly NFA dealer will walk you through your first time. (AHHH, the first NFA weapon). Mr. Keenan has the larger points. (Jim, you mean the CLEO sign-off isn't suppossed to be a bribe??? Hmmmm.).