Interesting. I always thought the Hydramatic was 4-speed.
According to the following there was a three speed version later:
https://en.wikipedia.org/wiki/GM_Rot...c_transmission
"Roto Hydramatic (sometimes spelled Roto Hydra-Matic or Roto-Hydramatic) was an automatic transmission built by General Motors and used on some Oldsmobile and Pontiac and Holden models from 1961–1965. It was based on the earlier, four-speed Hydramatic, but was more compact, providing only three forward speeds."