What 3spds manuals in 72 swinger?
No mounting pad for a floor shifter on a 903.
For that you need a 230 3-speed & then you will also have synchros.
Easy way to spot what you have, 903 is a top loader, bolt on top cover.
A body 230 has the bolt on side cover & a single mounting pad for a shifter.
The B & E body version has a longer tail shaft with dual shifter mounting pads.