That's wrong. You no longer need a third password in protonmail. All you need to have, in order to login, is the username and a password. If you've 2FA enabled, you need the 2FA code of-course.
I think you mean second password rather than third, but as a user of ProtonMail, I need one username, two passwords and one 2FA token to get in, with only login username/password being kept in a password manager (and all password managers get confused by multiple passwords, so I couldn't keep them all even if I changed my mind and wanted to).
ProtonMail may have the option (I am not aware of this) to have login password and mailbox password set the same (and not prompt you twice if this is the case), but they are still separate passwords. You, as user, control whether you want them to be the same or not. If you chose this, the application then has an option for convenience to use the same input for both tasks. This is opposed to a service where they are always the same, so that the password send to the backend is the same used to decrypt your data.
