Unless you can find someone you know giving them away, or selling dirt cheap; It'll *probably* end up costing you more to build a G3 from parts than it will to buy a second hand one. (Unless you want to go full-out and get a full metal body/RIS/custom furniture etc., in which case you'd only end up buying new parts and removing the old ones anyway)
Nonetheless, you will need: (Not an extensive list, as there's bound to be bits I've forgotten in the nine years since I last had a G3)
Upper and lower receiver.
Stock with the butt plate/pad
Wiring (front or rear, depending)
Outer Barrel, foresight & flash hider, cocking lever and associated parts to access the hop unit.
Hop Unit & barrel assembly
Pistol grip with motor.
Safety catches
Mag release parts.
That's all I can think of at the moment; but there's probably some more small parts somewhere I've forgotten about.
Edit: *And a foregrip; duh!