Most M15 3spd synchro are the same ratio, except for a close ratio one fitted to HT V8. 1st gear on the normal ones is about the same as an M20 aussie.

3.07:1 1.68:1 1:1 Reverse: 3.59:1

Where the 3spd is better is its reverse. You can get away with running a 3.08 (or even 2.78 behind a 308) then 1st will be about the same as M20 wirh 3.08 but reverse is lower, whereas all aussie 4spd have reverse the same ratio as 1st. You can use an aussie 4spd shifter with one too, reverse is where 1st is. This is how 3spd Bedfords were done. From memory I think the 4spd extension housing fits straight on a 3spd so the shifter fits exactly as per on a 4spd and uses the same shift rods etc. Only hassle is the reverse switch, it'd have to be something like the Bedfords used.