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It didn't have any cords or controllers, but I already have all those (it uses the same power and AV cords as the Genesis 2, and the controllers are identical).

It was actually a bit bigger than I thought it would be, bigger than the Firecore (but it plays better).

Supposedly the Genesis 3 can play Japanese games? I don't have any to test it with.

Yes, it can play Japanese games. In fact, it is region-free. Although you can't use the 32X or Sega CD (duh!) with it.

Another problem is that certain Genesis games use a certain bug that's in the first two models. The Genesis 3 is missing that bug, so you can't play games like Gargoyles on it.

For more info: CGR review: https://www.youtube.com/watch?v=oMhpgOt27fM
Gaming Historian: https://www.youtube.com/watch?v=2OGeUUmUAtI
32X incompatibility fix: https://www.youtube.com/watch?v=T3IZNCaLJSw
Unboxing: https://www.youtube.com/watch?v=EdOd20QwgGM

IIRC you also can't use it to play Virtua Racing, because it's incompatible with the extra chip in the VR cartridge for some reason.