Actually the chronology goes more like this.
SG-1000 - first ever sega console
SG-1000 II - same as before just updated design (sort of like what happened with Genesis)
SC-3000 - an SG-1000 with the optional keyboard built-in, came out at the same time as the SG-1000 II
Mark III - basically the third version of the SG-1000, identical hardware
Master System - an upgraded SG-1000 with MIDI module (which was removed for North America and Europe), it was completely backwards compatible with all SG-1000 software. In fact some was just repackaged in the new carts and sold again.