I think you must have your terminology confused here. Assuming the Pro3 is like every other DSI synth, what you should be seeing in the Mod Matrix is as follows:
Sources - Amp Env, Filter Env, Aux 1 Env, etc. (no specific stages specified).
Destinations - Amp Env Attack, Amp Env Decay, Amp Env Release, etc. (and so on for all the other envelopes).
Thus, you could apply, say, the Amp Env (in its entirety) to the Decay stage (specifically) of one of the other envelopes, in order to change the curve of just that particular stage.
An example of how that might be applied musically, would be to use a second mod-slot to map velocity to the amount of modulation above, which would have the affect of making the sound either snappier (if you use a positive amount) or fatter and rounder (if you use a negative amount) the harder you hit the keys. It would also be useful, in this case, to use a third mod-slot to apply velocity in redundancy directly to that same envelope’s Decay stage, such that you could then also control the length of the Decay/Release stage as the envelope’s curvature changes…
Percussion Synthesis 101.
Anyway, hopefully that helps clarify things for you. If not, perhaps I’m not understanding the question.
Cheers!