[nickec]

It is common knowledge converting a car to three wheels means it is legally a motorcycle.

So how to do this with a Gen 2 Prius?

Weld two rims together. Reverse the rear suspension parts to support the central dual tires.

Sure. No walk in the park. Yet also not so difficult.

Why do it?

You can aero mod the old rear wheel openings. Mileage goes up.

You might use the wheel well volumes for storage.

Insurance goes way down.

Possibly you can avoid smog requirements.

Finally, it will be a real conversation magnet.

P.S.: 2 tires on the now single rim is legally a third wheel and NOT 2 wheels. Per modders who passed DMV.