Here is all of the revelant motorcycle classification information for Michigan, which has no helmet laws. That being said, I still wear a helmet every time I go out in the M3W.
Found in this thread here:
http://www.talkmorgan.com/ubbthreads.php/topics/133102/1So, I was reading through the Michigan Secretary of State Motorcycle Operators Manual (MOM) in preperation to go take my test and get my permit so that I could register for the riding course to get the CY (motorcycle) Endorsement on my drivers license. I get almost all the way through the booklet, all the way to page 53 when I find this gem under the "Supplementary Information for Three-Wheel Motorcycles" section:
The following vehicles are not included in this definition, and therefore testing requirements may not be applicable. Please refer to the Michigan Department of State for regulations regarding testing for:
• Automotive hybrids or automotive conversions
• Vehicles with automotive controls or seating
• Vehicles with front- or rear-mounted engines (engines must be mounted mid-frame below the rider to be considered motorcycle-based)
• Vehicles with enclosed or semi-enclosed riding compartments
• Motorcycles or scooters with two close-set wheels in front (contact patches less than 18.1 inches apart) that lean and maneuver like standard, single-track, two-wheel motorcycles
or
• Vehicles with any other departure from the above standards.
This is what the state considers a 3 wheeled motorcycle that does require an endorsement:

Found here:
http://www.michigan.gov/documents/sos/Supplemental_Info_three_wheel_vehicles_325990_7.pdfSo, looks as if a Motorcycle Endorsement is not needed after all!