A flat "per mile" is a bad way to measure road consumption for a vehicle. The more accurate measure is weight of the vehicle+mileage. A heavier vehicle will do more damage to roads. There is at least some correlation on petrol/diesel because a larger vehicle will have to consume more fuel to move a heavier vehicle.
However, if there is a flat 3p tax per mile irrespective of the weight of the vehicle, then smaller/lighter EVs will be paying disproportionately more of the cost of road damage than larger EVs.