Dexron III is obsolete - it is no longer produced anymore. Dexron VI is the new specification.
Dexron III clones still exist, usually called ATF or D/M or similar. Read the back of the bottles.
Dexron II or IID is actuall the right spec for w123 and early w126 cars. Of course that was obsolete when Dex III came along.
Technically for all GM ATs, Dex VI should be used with confidence. However, we do not run GM ATs,
AFAIK, so that is part of the consideration. Dex VI is less viscous, but more shear stable. it starts at a viscosity level that dex III shears down to quickly, but Dex VI stays at one viscosity permanently.
There are users here that are running Dex VI without issues. Longevity would be my question, though at the time I have no reason to doubt. Dex VI is a very good ATF.
Redline sells a synthetic Dex II ATF. I have considerd at times going that route. May still in the future!
I personally are not a fan of a do it all ATF that meets too many specs ("Universal ATF"). But that's just me...