Oh, and to clarify (or possibly further confuse!) this:
Either model of SG-1000 [or the Othello Multivision or Taiwanese Aaronix system] will play SG-1000 games (anything with a G-xxx part number), but not the educational SC-3000 carts (those with an E-xxx part number) or SC-3000 Basic carts (B-xx)
Plug an SK-1100 keyboard into either model of SG-1000 and you can also play the E-xxx educational carts, but not the SC-3000 Basic carts. However, 3 SK-1100-specific Basic carts (B-21, B-41 & B-51) were released for use with an SG-1000/SK-1100 combo.
An SC-3000 (or SC-3000H) computer will play all SG-1000/SC-3000 carts EXCEPT for the 3 SK-1100 Basic carts.
A Mark III [or Japanese Master System or Samsumg Gam*Boy] will play all Mark III games and (G-xxx) SG-1000 games, but not the educational (E-xxx) or Basic (B-xx) carts.
Again, an SK-1100 can be plugged into a Mark III [but not a JP SMS or Gam*Boy] to play the E-xxx educational carts.
Oh, and SG-1000 MyCards (the blue ones) will play on any SG-1000 or SC-3000 through a CardCatcher, or on a Mark III/JP SMS. Mark III MyCards (the red ones) will only play on a Mark III or JP SMS.
Simple, right?
In case anyone's still confused, here's a handy table: