If you already have access to ShowingTime with another listing or Realtor, all you have to do is tell your agent to enable you for ShowingTime access on the second or third listing. 

All of the notifications, requests, and feedback will be available for the listings where you have access to through ShowingTime. 

You can see these listings in the Homes for Sale section of the menu

You can have multiple listings for sale with the same or different agents. You will be able to contact each of them and keep track of appointments and feedback.