Next button?

Any object (text box, shape, picture) can be given a Selected and/or Visited state. Those states will function just like the ones on a button. Thus, a trigger can look for the Selected or Visited state on those objects. 

Here's the problem: The trigger to change Next to Normal runs when a variable changes. But there's no trigger on that slide to change the variable, so the trigger will never run.

• If you change that trigger to run when the rectangles reach the Visited state , the Next button will change to Normal when the third rectangle is clicked.

Other things to consider:

The trigger to disable Next will run again if the user revisits the slide. To fix this, add conditions, so Next is disabled when any of the rectangles ≠ Visited. 

• It's more stable to use "≠ Visited" (is not Visited) as a condition, instead of 
  "=Normal" (is Normal). As Super Hero WaltHamilton  once put it: Like us, every state thinks it's Normal. In other words, Normal underlies every state, so the "=Normal" condition doesn't always work as expected.  

You don't need to have a text box on top of the rectangles. You can enter text directly into a shape. Just double-click it, and start typing. As needed, go to Format Shape>Text Box to adjust the margins of the shape and the text placement. 

Here are the best practices for uploading a .story file:

• Only include slides that are related to the problem. 
• Be sure objects, layers, motion paths, and variables have meaningful names. 
• If there is proprietary content, replace or delete it. For example, replace 
  proprietary text with “ipsum lorem” text. 

FYI: Here's more info about controlling Next: TIP: Controlling the NEXT Button 101 | Articulate - Community