This is likely the one you need if you want the M1/M3 version:
https://www.midwestgunworks.com/page/mgwi/prod/60516
If you want the comfortech, then here is the SuperNova one:
https://www.midwestgunworks.com/page/mgwi/prod/61176
Verify your M3 has the telescoping recoil tube before you purchase. Mine did (hence the ability to swap the SuperNova telescoping stock.) I almost bought one of these for my SuperNova, yet could not come to grips with spending almost $200 for a stock when I could by a complete SuperNova with the comfortech for $500.
I ended up putting the money towards a M2 with a field stock and love it. The M3 is an unique gun, yet the pump mechanism adds some weight. Something to consider based on how you intend to use it.