Dave Smith Instruments SYNTHESIZERS

It's not available because it's not needed.

Presumably you have ENV3 assigned to control some other parameter (such 
as NOISE LEVEL).

Rather than modulating the sustain level of ENV3, why not just modulate 
the parameter it's controlling directly?  In the case of NOISE LEVEL, 
modulating ENV3's sustain would be no different than modulating NOISE 
LEVEL directly.

Use the "Amount" parameter for your modulation source to control how 
much the level is being affected.

All of Evolver's modulations are "additive" - so if you point multiple 
mod sources at a destination, their effects accumulate.