To set up each mailbox for each of your TalkTalk Mail addresses you will sign in to each mailbox one at a time. An Internet browser handles only one TalkTalk Mail session at a time because it uses session cookies to authenticate the sign in. So when you've finished setting up the primary mailbox then sign out of that mailbox to end the session. You can then sign in to the second TalkTalk Mail address using its email address and password. Sign out of that second mailbox to allow you to sign in to the third using its email address and password.
When you've customised each mailbox you can, if you wish, sign in to the primary mailbox and via the Add email account function add the second and third email accounts to the primary mailbox sign in so that you see all three mailboxes and their folders and messages via a single sign in. Also via that single sign in you can send mail from any of the mailboxes.
Let me know if you need further guidance.
