The Genesis came AFTER the Megadrive I and Megadrive II. It was the last truly commercially successful Sega console. The Sega CD (and the CDX mini system) flopped pretty badly.
The Saturn didn't do so well either. Whilst it had the best sprite system available on a console at the time, it didn't have such good 3d support. Sony sucked up the market with 3d graphics. Toshinden, though hardly a good game, drew people to the Playstation, and the richer third party support kept them there. The Saturn did pretty well in japan (mine's a japanese one. I bought it the first day the cheaper clamshell black design was put on the shelves in Tokyo).
By the time the Dreamcast came along, Sega was already in the habit of handing their lunch to Sony. Even with a year's head start, Sony's Playstation 2 walked on the playground stepped on the Dreamcast and sent it packing.
Its a good thing that Sega got out of the hardware business: they simply don't know how to do it. They never marketted the Saturn or Dreamcast well here in the states, nor did they court third parties as aggressively as they should have.
