I have a good Saginaw 3 speed and a good Hurst competition plus shifter with the correct mount. I don't see why this couldn't be done. Has anyone done this before? I may go to a 4 speed in the future.
Move the reverse lever to one end and tie it off so you don't accidentally get into the wrong position at an inopportune time.
I ran the same combo for several yrs. and never tied anything off. Never had a problem. I don't guess it would hurt anything, but don't think it is necessary.
Uncle got a good deal on a car that had a "bad" transmission once. Turned out it had a four speed shifter and knob on a 3 speed. Seller kept trying to shift according to the pattern on the knob and it would be of course reverse instead of 1st.
I've seen 3 speed shifters used on 4 speeds in drag cars years ago; it makes missing 2 gear less likely as the levers ARE moved through the reverse "notch". A separate lever has to be fabbed up for reverse, and you want to be careful not to have it in first and reverse at the same time (BOOM!). Just the opposite of what you're using/setting up. No reason it should't work; maybe you could even make the reverse lever part of the starting circuit with a switch, that could also serve as an anti-theft device if a 3 speed shift knob/ball is used to confuse a thief. I am Butch/56sedandelivery.
Saw one like that myself on a 64 Chevy one time. They used a cut off lug wrench for the reverse lever. Looked goofy, but it worked.