I was a seat belt design engineer at GM for several years
What you are doing with a 3-point system is extremely dangerous if you get into a crash.
The dynamics of the human body in deceleration with a 3-point system is much more complex than with a simple seat belt.
During the design stage of a new system in a new car model, countless tests are run to assure that no "submarining" does not occur. That is when, during a crash event, the chest belt does not allow proper forward movement and the torso sides under the lap belt and decapitates the occupant.
Take it for what it's worth, but I had observed dozens of crash tests that ended with that result.