Well, I learned something new today!
I never knew the SG-1000 was its own thing; as someone not that familiar with Sega when I always thought it was kinda like the Famicom to the Master System's NES, a JP only version of the same console that looked different and released earlier. Cool stuff.
Although if we get technical we could also argue for stuff like the Colour Game TV for Nintendo which muddy the waters still further.
I'm gonna let the poll and such stay as is for now, so as not to wipe the input received so far. We can just consider it third major console as stuff like SG-1000 and Colour Game TV didn't see global release.