Nice. I note that the original owner
You may well be able to make use of something last seen online as:couldn't find the original 3.5" software diskette. (This mouse) will still function great as a single-button mouse.
three-button-power-251.hqx
The Read Me First! file associated with the control panel found in the above item states that it's the
.Macintosh A3 Input Devices3-Button Power™ Control Panel
I have a copy locally, and am willing to send it to you if you can't find it elsewhere.