[Figment]

Depends on their armour vs firepower. 3 men tank should be equal to three one man tanks (Lightning) if not slightly better.

Which would probably mean that it would be crewed by two anyway to get more endurance total...

Hence I would go for two crew on a MBT (driver + gunner) with gunner controlling all weaponry and save three crew on a very heavy tank if they ever add that to each empire (driver + 2 gunners). Benefit of two crew would be all gun control for one player and more battlefield efficiency and a bit more survivability over a 1 crew vehicle.

A three crew vehicle should have high firepower per gunner (dedicated roles) but also severe weaknesses like for instance turret rotation speed, overall speed and poor gun depression angles to ensure there's both advantages and disadvantages to playing them. There should not be a default choice, but a situational choice to pick one of the three tanks if you have three people. To ensure there's more than one person in a tank with high endurance and they don't become Epic Lightnings and Lightnings are a gameplaystyle choice, IMO never give the gun to the driver if there are more people inside unless it's a peashooter (like a fixed frontal AI gun) for defensive purposes.