The great majority of the 3 speeds were the light duty box with a 4 bolt side cover. These are internally similar to the passenger car transmission, but use a different case. If you have this transmission, it should look like the pics on this page (disassembly and repair instructions are here too, if you need them):
http://chevy.oldcarmanualproject.com...4ctsm0705.html
That HD transmission shown in the preceding post is a Warner T-89 and is fairly scarce. If you actually have one of those, consider it a prize. Most buyers looking for a transmission upgrade went for the 4 speed because it didn't cost terribly much more than the T-89.
Ray