Well, this thread made me curious so I checked the portuguese legislation. Since all european legislations have become similar I suspect this solution must be the same as in many other european countries:
Here goes, sorry but it's a google translation of the law:
1 - The driver and passengers in cars are required to wear seat belts and other safety devices with which vehicles are equipped.
2 – Separate regulations will set:
a) The exceptional conditions of exemption of the obligation of use of the devices mentioned in the previous number;
b) The method of use and technical characteristics of these devices.
3 - Drivers and passengers of mopeds, motorcycles with or without sidecar, trikes and quads must protect their heads by wearing officially approved helmet models, properly fitted and tightened.
4 - Provisions of the preceding paragraph do not apply to
drivers and passengers of enclosed vehicles or vehicles that have simultaneously a rigid protection structure and seat belts.
So ... as per #3, trikes are to be used with helmet. However, #4, I suspect worded to comprehend the BMW C1 bike, also applies to tricycles. And the M3W has both a rigid protection structure and seatbelts fitted (not sure if "enclosed vehicle" applies to the M3W but it becomes irrelevant because the other two conditions are met)
Oh and definitely, according to portuguese law, the M3W is NOT a car, number of wheels dictate the category.
