There definitely should be if there isn't lol. That could be very detrimental to gameplay if you go halfway across the map and run out of ammo only to be short changed by a drop sitting in a pond haha. Maybe another option could be that one of the ammo/health/shield drops is set to drop from an enemy and there's already 3 out in the world the one set to drop would replace one of the other ones out there and that other one would disappear.
edit: My friend just brought up a point to me. While we were playing there were drops left behind but when we finished another fight there were also drops that would add up to more than 3 total for each kind. Sorry if I'm making this confusing lol I'm just curious as to how you guys worked it out because I live that no ammo Ranger life /cry