MacBook Pro showing Username and Passwordfield though unencrypted
Have you turned on FileVault 2 encryption? If you have, you can enable your third account by doing the following:

Open System Preferences
Click on Privacy &amp; Security.
Click on the FileVault tab.


There should be an Enable users... button showing in the FileVault preference pane. Click it and you should be given the opportunity to enable the third account by providing the account's password.

On restart, the third account should now appear at the login screen.