2 (& 3): Used both, prefer both (and plate) if lots of lead. Prefer weight belt if not neutral.
If I were neutral: no need for the weight belt.
No suit: Hmm. neoprene belt with ~5lb. Never dove that warm though. Or just plate.
Thin: high on plate to max out belt, weight belt.
Thick: high on plate to 'max' out belt, 6-12 lb. weight belt, integrated that easily removes from rig.
all with crotch strap that is D-ring and tri-glide free.
Tiny doubles: same, but they're no different than a single
Sidemount: spine pockets, shoulder and rear hip trim/ditch pockets, all on harness.