Sonic the Hedgehog 3 was first planned around which Sega enhancement chip?

Sonic the Hedgehog 3 was first planned around Sega's Virtua Processor chip.

Development began in January 1993 at Sega Technical Institute, where the team initially explored an isometric, three-dimensional Sonic game. The Sega Virtua Processor was intended to support the prototype's polygonal graphics, including an early special stage featuring a 3D Sonic running through a figure-eight course.

The chip was not ready in time for the planned 1994 release, and the developers also concluded that its low polygon count limited the experiment. Sonic 3 was therefore restarted as a conventional 2D platform game, while the isometric concept later influenced Sonic 3D Blast, released in 1996.

The Virtua Processor should not be confused with the Yamaha YM2612, which was the Genesis sound chip, or with Nintendo's Super FX, which powered 3D effects in some Super Nintendo games. The final Sonic 3 cartridge did not use the Virtua Processor; it retained the Genesis hardware and its familiar side-scrolling design.