In Austria the MTW is classified as a 3wheel motor bike.
Here wearing a helmet is mandatory for bikes...

Other countries, other regulations:
I'm interested if MTW owners drive with helmets or just use goggles and maybe other headgear. What's your best experience?

1. Full face helmet in cold weather/rain.
2. Open face with pop up visor, spring and autumn.
3. RayBans/ no helmet, summer. Safety glasses are also a good alternative.

1 & 2 are for comfort with an added safety element built in.

+1
Also use helmet on the motorway.
Ear plugs otherwise...

With my 4/4 & +4 with aeros I always used either goggles or safety glasses. A bee in the eye at 20 mph on a bike in the 70s taught me not to risk no protection.

mine is on order, but I'm not planning on wearing any helmets. just motorcycle glasses and goggles. Im in South Carolina in the states, with the roll hoops and sitting in it, you don't need a helmet. But it is a grey area in motorcycle law . it runs like a cycle but drives like car. your in it with seat belts,and roll hoops.

I've got Schuberth flip up C3 helmets with bluetooth intercom form my bikers time.
Do I need special car racing helmets or will the bike version to the trick? Most likely there is more air ventilation on the MTW than on my Honda GoldWing.
I have Arai jet helmets too but without integrated eye protection and sunshield.

Pilot goggles are available...

Shoei j-cruise is my helmet. I recommend it.
Fully water proof Flip up perspex face visor. Also available is a double glaxed section for cold days.
Pop down solar lens .
There are variable cooling ducts too.

Shoei and Schuberth are similar quality class.
I had them both and finally the Schuberth fitted beter to my head.

I passed through Austria last year with the Three Wheeler.

I was wearing a Biltwell helmet, but just because I was driving on high way for several days.

https://www.biltwellinc.com/c2/buy/helmets/

Cheap, light and slightly vintage style.

You can find also the Davida collection or chase some used vintage helmet... I don't think the police would go too far in the compliance check.

My wife and I each use a Caberg "Downtown" open face helmet with flip up visor.
These provide good warmth and weather protection in winter without being too hot in the summer. They also make the bark of the exhaust bearable on a long journey and have the added advantage of having a built-in tinted sun visor that can be swung down in front of glasses if needed.