Use of triggers and variables

The Visited state comes with built-in functionality. An object with a Visited state will automatically change to that state when it is clicked. 

Here's how to do what you want:

• Replace the Visited state with a custom state. For example, you could call it Completed. 
• On each layer, add a trigger that changes the corresponding button to the Completed state with the layer's timeline ends. 
• Enable the NEXT button when the state of the 3 buttons  = Completed. 

FYI: This post had a file that demonstrates various ways to control the NEXT button: TIP: Controlling the NEXT Button 101 - Articulate Storyline Discussions - E-Learning Heroes