Aux and Matrix mixes are not designed to feed into the Main Bus.
This is exactly what a Matrix IS for.
A Pre-Fader Matrix Mix is a buss that gets its signal from other busses, instead of inputs (like an Aux). *** Series 3 Matrices can source inputs for it's Matrices, which is a unique workflow to the Series 3***
You send the Main and any Aux mixes into a single Matrix.