I don't know much about GM 3 speeds, but Chrysler had two three speeds during the '70's. I/ve had both in cars/trucks. One was the A250 which was no-synchro on 1st gear and the A-230 which was all synchro on forward gears. A230 worked great on a Duster I had with a modified 318.
FWIW I found this list on Wikipedia:
Saginaw M26/27 transmission — 3 and 4-speed longitudinal light duty (less than 300 hp) wide ratio manual transmission made by GM at their Saginaw, Michigan factory
Muncie M62/M64 — 3-speed longitudinal transmission made by GM
Borg-Warner R-10 overdrive - 3-speed manual transmission with electric overdrive used 1937-1964
Borg-Warner R-11 overdrive - 3-speed manual transmission with electric overdrive Ford used them up until 1975 in trucks.
The entire list of all trannys is here: (I culled out the 3speeds)
https://en.wikipedia.org/wiki/List_of_GM_transmissions
FWIW