This is all wrong.

> multijunction cells can exceed 50% efficiency

The maximum demonstrated efficiency of a multijunction cell, in a lab, WITH CONCENTRATION is less than 50%. Commercially available cells are lower.

Concentration is an important caveat for two reasons:

First, it implies that you are collecting light from a larger area than the PV panel itself. Second, efficiency grows with increased irradiance (so efficiency will be lower without concentration).

> 3 square meters of lateral area

Lateral area is meaningless. It’s all about area perpendicular to the solar axis. Unless you are driving a box van or a big pickup truck, there is zero probability that you can put 3 kW of irradiance on your panels. Neither of those vehicles will achieve kWh/mile numbers anywhere close to a Prius.

In practice, you need to halve the efficiency and more than halve the collection area you quoted. You also need to account for conversion losses.