title: change background color of all view controllers if all classes are called (swift4)
body: <p>When the user visits all three view controllers in no particular order. I want all of the view controllers in this program to turn green. But only if all three classes are visited.  I don't know if this is a coredata or userdefulat thing. </p>
  
  <pre><code>import UIKit
  
  class oneV: UIViewController {
      override func viewDidLoad() {
          super.viewDidLoad()
  
      }
  }
  
  class twoV: UIViewController {
      override func viewDidLoad() {
          super.viewDidLoad()
      }
  }
  
  class threeV: UIViewController {
      override func viewDidLoad() {
          super.viewDidLoad()
      }
  }
  </code></pre>