Not letting the user progress until they have completed all tasks

Hi Everyone,

I'm quite new to Articulate Storyline and E-learning in general and I've found this forum very helpful so far as I'm mostly teaching myself. I just thought somebody might be able to help me?

Basically - I'm working on a project that has 3 case studies in it. I have built a slide that takes the user to 1 of three scenes depending on which image they choose. After the user completes a case study, they jump back to the slide that'll let them choose the next case.

My aim is to not allow the user to progress until they have completed all 3 of the case studies. 

I have tried to set up triggers and variables to hide the next button until all three images have changed to "visited" 

I think what I have done would work if it is all on 1 slide, but as I have used 3 scenes, the states of the images reset to normal when the user returns to the slide. 

Is there anything I can do to fix this? Sorry if I made no sense!