Two-factor authentication (2FA) is much more secure than just a password because it uses two different authentication methods to prove identity. If you use 2FA, the third person will be unable to access your online account if he obtains one of your authentication elements (password). Your data will be safe as long as he does not have access to the authentication device (a smartphone or tablet).
You'll need an app like Google Authentificator for iPhone un iPad or Android phones and tablets or Authy to set up 2FA.
We recommend that you use the application's option to generate recovery backup codes in case your device fails or disappears, and that you keep them in a location that only you have access to.