Great news - I figured this out and installed a 12" subwoofer running off the M3 amp. Turns out there are actually 4 wires going into the 8" sub, not 3. Those 4 wires are because it's a dual coil subwoofer running 2 x 80w at 4ohms. This matched the MTX 12" sub with box I had in the garage.
The colors going out of the amp to the sub were different than the ones inside the sub enclosure.
They were:
From Amp:
Green, Red, Purple, Brown
And were changed to
Green, Black, Red, Blue
Green to green (pos)
Red to Blue (neg)
Purple to Red (pos)
Brown to Black (neg)
Rather than removing the sub and creating all kinds of drama, I just disconnected the connections to the speaker itself and ran extensions from the sub enclosure to my speaker box.
I think that the sub is running off a series to the door speakers because there's a sound change for sure. As time allows I'll update. For now, I have substantially more bass and the sub hits well. Not as well as having it's own dedicated higher wattage amp, but I like to keep my teeth in these days anyways.
Hope that helps!
j