The point is that the combination of the three is the most powerful and secure. To copy something-you-are, they have to have access to you or things you've touc

The point is that the combination of the three is the most powerful and secure. To copy something-you-are, they have to have access to you or things you've touched. To copy something-you-have, they have to steal the object itself. To copy something-you-know, they have to trick you into revealing it (phishing) or guess it. Requiring all three makes things that much more secure.