The Diamond DA-40 that I fly has a three-point harnass that passengers always take to be exactly the same as a car-style harnass, but it isn't. The lap strap is completely separate from the chest strap. They just share the same buckle. But the lap strap needs to be tensioned first and stays that way. The chest strap is on a reel. So the lap strap prevents you from going up and down, the chest strap prevents you from going forward in a crash. But indeed nothing prevents you from going sideways.
For aerobatics, I agree you need a four- or even five point harnass.