DEX III Has been obsolete for years. I believe in 2005 maybe 06, there was a service bulletin that states DEXIII is out and DEXVI is in! for various reason but mainly because DEXVI is a far superior fluid that helps repair the soft parts inside... friction modifiers. the service bulletin was strictly for the fluid and the vehicles that use it. Therefore I personally believe it is also used in TCases... and has been in mine for years.
Al
There was a GM TSB which stated not to use Dexron 6 in certain applications that called for Dexron 3. Specifically transfer cases, manual transmissions and power steering systems. @MIHELA explains it well in the quote below. He and others also mentioned in another thread that the different viscosity between Dex 3 and 6 is probably the reason for this.
GM specifically said not to in a bulletin. That's why they kept "Manual transmission and transfer case fluid" which was Dexron III under a different name. Technically they couldn't call it Dexron III because they stopped certifying that spec but its the same.
Edit: the bulletin is #04-07-30-037D. The notes about manual transfer cases are towards the bottom of the first page:
"The content of this bulletin does not apply to manual transmissions or transfer cases. Any
vehicle that previously required DEXRON®-III for a manual transmission or transfer case
should now use P/N 88861800. This fluid is labeled Manual Transmission and Transfer Case
Fluid. Power Steering Systems should now use P/N 9985010 labeled Power Steering Fluid."
That said, I've had Dexron 6 in the transfer case of my Suburban for a few years now and I haven't observed any negative effects. I found out about that TSB after I'd already replaced the fluid with Dex 6. I bought some of the GM recommended 88861800 fluid mentioned above with the intention of replacing it after I found out, but haven't done it yet. So far, so good.