Its the same reason breachers are not allowed up front.
3 Gun only seems unsafe because it is the ADHD of action shooting.
If you can't shoot one gun that well, you put it down and pick up another and keep running.
These shooters are easily distracted with the mechanics of keeping their guns loaded. No need to be all over them if they have to turn their muzzle at you when they drop their guns all over the place.