Older GM 2WD trucks did indeed come factory with three speed manual transmissions--they used Muncie,Saginaw,and some even had Tremec 3 speeds,I had a Tremec in my '75 2WD Blazer when I first got it,it had a 250 six with that behind it factory..
I recall almost breaking the bell housing off trying to pry that transmission free to replace the clutch--it was then I learned a Tremec had a 5th bolt up on the top,instead of the usual 4 bolts to attach it to the bell housing..
I put a Saginaw 4 speed in it eventually from a '75 Chevy Vega,it was practically a bolt in deal,all I had to do was use the shifter it came with and carve a new hole in the floor hump for it..
I know GM used some "top loader" Ford 3 speeds in some 60's GM full sized cars,like Pontiac Tempest/Lemans,Buicks,and Oldsmobiles-,they were not common,but did exist---rumors say some 2WD 1/2 ton pickups had them too,but I've never seen one..
Strangest "factory" tranny/engine combo I've seen,was a 1961 ,1 ton 2WD Chevy Suburban,it had a 235 straight six,and a factory Clark 5 speed trans,that was about the same size as the engine!..it also had torsion bar front suspension,and everyone thought that was "new" when GM reintroduced it on the '88 & up new style trucks !..
The truck also had a huge rear diff,looked like an Eaton,and very low gearing,at least 4:56's or lower too..