It actually *is* possible, but takes more work on our part (and no, it really shouldn't be up to us). But you can do SSS even using all 3 voices, but it's *very* complicated. You have to match the internal volume of each part, additionally, to avoid sample buffer flushes or sudden DSP switches (which can make quite a loud pop) you probably want to use a legacy or gm/xg as one or 2 of the 3 voices (fun fact, if you use all 3 voices as GM/XG, you don't even need to do any special programming in that case! They *all* used to be SSS until around the 2000's)
Long story short; just assign R1, R2, R3 as your different voices that you turn on and off, would be the simplest.
Mark