A three-point harness is not really all that safe and superior. To put it in terms familiar to most people, a three-point safety device is a typical safety belt.
Now that we have established what is familiar to you, let us talk about something that is not necessarily so.
A 5-point harness is something you can see in racing cars. That should be expected, after all, child safety devices were inspired by racing gear to begin with. That makes perfect sense. Both racers and children need perfect protection in case things go awry. The former because of the way they are expected to drive, the latter because they are simply fragile.
A 3-point safety device simply keeps the passenger in place. A 5-point harness hugs them tightly, preventing submarining, injuries delivered by the safety device itself, misplacement of the straps and so on. Additionally, more straps mean that the pressure in the event of a car crash or a sudden deceleration is better distributed.
Do note that booster seats are effectively three-point safety devices because of the way they function: by guiding the vehicle seat belts.