In the state of Nevada all three wheeled vehicles are considered a Tri-Mobile. The state recently updated the state helmet law regarding Tri-Mobiles and the updated law goes into effect Oct 1, 2019.
The law now states that if the three wheeler has a saddle or handlebars you must wear a helmet (Can Ams, Harleys, Hondas etc.). It goes on to say that if the vehicle is fully enclosed and has a steering wheel, you do not have to wear a helmet (parking control vehicles). The law is non-specific regarding the Slingshot so clarification was sought from the state. Per a rep from the state, Slingshots are exempt. You do not have to wear a helmet in Nevada.