it is confirmed, by many. the Genesis 3 does not have the z80 chip.
And how did "many" come to the conclusion that it's missing? The indications listed on the linked page are dubious at best:
1. Unable to run the MS convertor as it needs the Z80 chip which is the brain of the SMS.
The Nomad can't run in SMS/M3 mode either. This has to do with certain VDP and bus arbitration capabilities and does not require the Z80 to be missing by any stretch of the imagination.
2. Unable to run Game Genie, the Z80 must have played a role in the use of this for code break ins.
This statement is logically pretty broken; combined with the initial line, it basically says that Game Genie must use the Z80 because Game Genie uses the Z80.
3. Unable to run special games such as Sega's Virtua Racing, which again uses the Z80.
If Virtua Racing doesn't work, it's probably due to missing or altered signals on the cart slot; the chunk of DRAM and coprocessor on the cart almost certainly have more to do with any compatibility issues than some unspecified "special" use of the Z80.
The rather glaring problem with this text is that it seems to be based on a perception that the Z80 is some archaic relic of past legacies untouched by all but the most privilieged Genesis developers, when in fact it's at the very core of the Genesis sound system and interfaces quite a bit with the rest of the system. If the Z80 was missing, it would be more than a few "special" games that didn't work.
It is likely that the Genesis 3 doesn't have a "Z80 chip" as such - there's no sense in putting something like that on its own chip if system-level integration is feasible. This does not mean that there is no Z80 any more than your PC not having a "serial port chip" means that it lacks serial ports.