How do you revoke one of three POA's?

If "the whole family" includes the person who has the unwanted POA, that person can resign it. But I agree that it would be better to get this situation sorted out under authoritative supervision, because only the person who gave the POA can revoke it and it sounds as though that person doesn't really understand the process - how else did you end up with three of them?

A POA is revoked when the person who signed the POA signs a second document revoking the Power of Attorney. As was said above, the person should be competent to understand the impact of the action, and may also have to directly advise the agent under the first POA for it to be effective.

You go to a judge and let him decide on a fair and impartial basis.

If the person who gave the POA is still competent to do so, they must go see the lawyer who wrote up the POA, tell them to write a new one without that person's name listed as a POA, and send them a letter notifying them of the change.