How to figure gear ratios in three speed tranny?

If you can't figure any other way, simply "rig" a way to turn the input---clutch disc, or vise grips and a rag, etc and some way to mark output shaft---yoke

Then just turn and count the input until the output turns once

To be more accurate, count the input until the output turns TWICE or FIVE times, then divide your answer by 2 or 5

Here's a page speculating on ratios

http://www.moparchat.com/forums/showthread.php?t=115907

I really have not paid much attention to 3 speeds. I broke enough Chivvies when I was a kid