Set keyboard control differently in 2 scenes to control which parts sound.

Scene 1 = part 1,2; scene 2 = part 1,2,3

1. Set Keyboard control ON in each scene for the parts you want to sound. For Scene 1 set control ON for parts 1,2 and OFF for part 3 and in scene 2 set control ON for all 3 parts

2. Add control assignments for Part Volume to AsgnKnob1 for each part 1,2,3

3. Add control assignments for those 3 asgnknobs to common knob 1 for super knob control

4. Adjust the volume of each part 1,2,3 manually to the minimum volume you want each part to have. The super knob can then control the volume up or down from there

5. Modify the super knob common knob 1 control range to set the min/max values for the knob. See p.45 of the reference doc for how to set the min/max knob values.

See this article for how to 'morph' between two levels

https://yamahasynth.com/learn/modx-series-synthesizers/super-knob-morph-mastering-modx/