Not necessarily what you'd probably call a benefit, but a big reason you see a lot of small companies like this, and arcimoto, and the old reliant robin only having three wheels is that they aren't legally considered cars, so they don't have to meet the same crash regulations, and there can be tax/registration advantages. That's why the Solo can even be sold without an airbag.

The advantage, from the buyers standpoint, is just lower cost. In addition to the reduced cash safety requirements three wheels just saves some money on and additional wheel/tire/suspension and potentially drivetrain components. Potentially you get some improvement in fuel economy as well.

Also lower rolling resistance, so better mileage.