there was so many things wrong with the M3 if you came from the M or M2. The AF was as bad even though Canon said it was going to be better. The sensor was okay. it was the sensor gen before DPAF, and used phased detection rows that could show up. The AEB took 1-2 second in between photos meaning no matter your AF, it took 3+ seconds, the sensor was completely unable to be IR converted (which for me was a $1000 mistake), the UI experience was very clunky coming from the M / M2. There were shutter timer features missing. There were color cast issues in the corners with fast lenses. Coming from the M or M2 it felt like Canon got out the cripple hammer, dialed it up to 11 and beat the damned thing to death.
We were all waiting for it, it wasn't released in the USA (obviously the one time you can state Canon USA was smart), so we all had to order it via Japan, we had online tutorials rolling on dpreview on how to navigate third party shippers to ship to USA / Canada / etc.
and that's what I can remember off the top of my head something like 10 years after the fact.
Canon could have corrected most of that via firmware but instead they did the M6 and M5 cameras that fixed all those problems and delivered a far superior camera(s)