if the user has clicked all 3 buttons, then go to the next page

Check the Navigation in the sample at this post:
https://community.articulate.com/discussions/articulate-storyline/free-sample-restricted-and-free-random-navigation-using-variables-and-triggers 
It has four buttons that take the learner to different slides, and when they are all visited, a button appears that can have any trigger on it you want. In the restricted navigation, the four buttons appear in order, only after the previous one is visited, in the mixed navigation, all the buttons have to be visited, but in any order. The free navigation allows continuing without visiting all the options.