You can link all three accounts together, so that you can login using whichever is most convenient.
Account linking is especially useful for someone who has registered for Pokémon GO using a Google Account that they may not be able to access in the future, such as one associated with a work or university email address. By linking their Facebook account, they can continue to make progress on their Pokédex, and even link a new Google account to their Trainer profile.
To link an account, go to the Settings menu and then select the platform you want to link.
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