The rearview mirror (the one that hangs off the windshield) is for seeing directly behind you. Your side mirrors are for seeing things to the right and left of you. If the driver directly behind you can see your face in your side mirrors, or you can see their front windshield, and your rearview mirror both exists and is unobstructed, then you are driving around unreasonably blind to vehicles overtaking you. This blind spot can be almost completely eliminated.

The side mirrors should be positioned thus that you have a clear view of the lanes next to you, with the door handle or body of the vehicle just slightly out of view. You can lean and tilt your head if for some reason you prefer a view of what’s going on behind you that is half obstructed by the vehicle you are currently driving.

The goal is to maximize the area covered by the mirrors so you can see more things, not to have redundant views of the same thing.