i have replaced the passenger side mirror on my 2019 CX-3. Mine has the blind spot monitor. You need to make sure the replacement is the same.
search Google for the mirror. Make sure you are buying the whole assembly, not just the casing, or not minus the mirror. OEM is really expensive but there are after market suppliers between $105-$130.
You need to take the door panel off And mirror removal. Check YouTube for some videos on this. There is a power cable that plugs into the old mirror. This controls the mirror adjustment and blind spot monitor.
Take special note how the cable goes under the window and door frame up to the mirror. It is difficult to route it through and not have the window hit it when you roll it down! i could not find a good video for that and it is hard to do.
I would say it took me about 90 minutes but I got it replaced and all works fine.