Everything went fine (mac and M3 firm-121a- camera recognised) but it says "no official build is available for this camera"
Right, I forgot to mention that. 121a is the latest firmware for the M3, but there is no CHDK port for that. You should be able to downgrade to 120f, using the official Canon firmware upgrade. Unfortunately, it seems that the Canon support sites no longer offer 120f (1.2.0 as they call it) for download. I
uploaded it to sendspace, link expires in 30 days.
You'll find a file named M3_120.fi2 in the zip, copy it on a card, use the camera's firmware menu to downgrade.
If you don't want to downgrade, a script-based alternative solution can be created, but that requires a different card setup and always has to be started manually.