Come & Knock On Our Door (Three's Company)

What is the title of the 1977-1984 sitcom starring John Ritter that was about three roommates with a theme song that starts, "Come and knock on our door. We've been waiting for you."?

Who are the three characters who make up "Three's Company"?

Who is the actress, and fitness buff, who played the ditzy, blonde character, Chrissy?

The apartment Chrissy and Janet lived in was not permitted for co-ed living. What did they tell the landlord to allow Jack to live with them?

What was the name of the Three's Company spin-off show, centered around the landlord's new life in a affluent community?

In what city was Three's Company set?

Jack was an aspiring chef and Chrissy a secretary. What was Janet's profession?