@whitequark IIRC AA3 also improved the damage model to account for limb vs torso hits having different effects on your performance (e.g. a leg injury, even if treated, reduces your movement speed while an arm injury impacts your accuracy more). But I could be wrong, I never played 3 much.
2.x had a single global HP value, the only time the damage model accounted for location was when calculating damage caused by a hit.
Oh, and grenades actually did (range limited) hitscan modeling for individual fragments to calculate damage, including reduction of damage by cover slowing down/blocking them.