So basically I want to simulate a tricopter, which is basically a helicopter with three blades arranged at 0, 120 and 240 degrees. So I need a way to apply a force from each 'propeller' downwards, that acts realistically on the object (i.e. applies torque, as the propeller are a distance away from the center).
Basically all you need is Rigidbody.AddForceAtPosition. But be warned: the logic to keep such a thing stable or rotate it in a certain way is a real pain. Even in the real world. Such helicopters are pure hightech ;). The mechanical concept is even much simpler than a normal helicopter but the stearing isn't.