How do I get more email accounts to show with my primary account?

There is a box which shows 2 of my 3 email accounts when I open my email. To check the 3rd one, I have to sign out and re-sign in. I would like to avoid this step. Thank you.

How do I get more email accounts to show with my primary account?
Sign in with the 3RD one so it WILL BE ACTIVATED.