If it has a 9 bolt side cover it is a Warner HD 3 speed. They are about the same size as the SM420 4 speed and low gear is really low. Good HD trans but not very common or popular.
If it has a 4 bolt side cover with a long tail shaft - total length of the trans is 22", it is a regular 3 speed also known as an SM318. Lots of them out there.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine