What is the difference between Dexron-II and Dexron-III?

The ii and the iii are refering to the year it was made. Both of those the dexron 2 and 3 are pre 2014. Dexron is the brand name of transmission fluid manufactured for GMC vehicles. Today we live in a world that offers higher quality transmissions fluids, including synthetics. Today we are up to dexron vi ,6