Change a button state to Normal when other buttons become Visited

Hi--

I am working on a presentation storyboard with a menu page. Three buttons are linked to three parallel slides covering different elements of the topic. Each of these slides links back to the menu page, where the corresponding button's state has meanwhile been changed to Visited. 

There is a fourth button, labeled "Conclusions" that starts out as Disabled. I want to change its state to normal (and have it stay that way) once all three buttons are Visited. In other words, I'd like the button to be unlocked when the user returns to the menu from their visit to the third page. 

I've tried this a few different ways and have not been able to figure out how to get a trigger to "fire" upon the user's return. 

I appreciate any help, will upload my latest attempt.