MS3 does not support a manual mixture control directly but it appears possible using the table blending feature. Not much information available about that feature but should be easy to test.
The manual mixture is not supported directly, but can be added. As I've noted before, put a potentiometer in the throttle control, and configure the cable to open the throttle fully at half travel. The second half of travel controls mixture.
I found the documentation for the table blending feature to be adequate. I don't see how the table blending would be useful for mixture control (not saying it isn't, just that I don't see how). Its intended purpose is to change the type of control between low and high RPM mixture control.