We've holidayed in Ravello on the Amalfi Coast quite a few times and many of the locals on scooters in that region at least definitely don't wear helmets! Pretty sure I've seen guys on full motorbikes without them as well but that may say more about the compliance culture in Southern Italy......
Scooters or bikes without helmets: An Italian friend of mine, living in Milano and earning his incom as an attorney, was caught by police without a helmet on his scooter. His bike got confiscated for a year and he had to pay a substancial fine. So be careful!
Live in southern Italy is completely different, but also there things can be expensive if worse comes to worse.
I would not mind at all to drive the M3W without a helmet when using seatbelts. It's like driving a convertible . You do not need a helmet driving a Piaggio Ape either (the famous Italian 3wheel mini-truck)
