QUOTE
"Poly Kbd retriggering mode should do what you're asking for, no?"
No, when you trigger a chord with three voices at the same time, the LFO's of all voices will start at the same phase in the LFO waveform.
When you assign the LFO with Poly Kbd to the pitch, all three voices will have the same pitch drift. Only when you trigger the three voices after another, the LFO's will be different.
A random retriggering option would solve this.