Do you mean that ST3’s signal going to Aux 9/10 should follow ST3’s main fader?
Set the ST3 channel in your AUXes to “Post-Fader” and set its AUX fader to 0 dB (..or adjust to taste).
This way the Auxes where ST3 is Post-Fader will follow the Main L/R fader.
See SQ reference manual, Chapter 4.7 “Basic Workflow Concepts”:
“Mix sends set to ‘Post-Fade’ follow the LR send levels. To toggle channels
between ‘Pre-Fade’ and ‘Post-Fade’ for the selected Mix, hold the ‘Pre Fade’ key
and use ‘Sel’ keys.”
Also cf. Chapter 8.4 “Send Levels and Mix Assignments”.