I think that helmets in a trike will look rather odd.
A leather flying helmet & goggles is a different matter - Rocky & Bulwinkle style.
Over here in Australia, there are many 3 wheeled motorcycles called trike's, 2 very large rear tyres and lots of chrome. Usually powered by a Harley Davidson engine.
They have to wear helmets regardless of what they think.
How can a 3 wheeled (car) be anything else? calling the new Morgan 3 wheelers by any other name is just fooling oneself. When tumbling down an embankment at 60 k's I would definitely be glad of a helmet.
More boys are killed here riding quads than motorcycles, next is girls falling of horses.
Its not mandatory to wear a helmet on a Quad on private land.