FYI, Lee also make glass filters for their system; I believe that they are made to order.
But those are solid NDs, not graduated. AFAIK, Schneider (and now their subsidiary, B+W) are the only ones the make grad NDs in glass.
Is 3 stop really necessary NA? I live in a perennially sunny part of California where a ND is required for landscape shots, I'm getting good results with a 1 & 2.
It's not really about how much knockdown you need overall, but rather the exposure difference between sky and ground. Most of the time, that's in the 2-3 stop range (often higher). I can see combining a grad ND with the Big Stopper, for example, to achieve a long exposure but also allow more of the scene DR to be captured.