Making a button appear after visiting certain scenes

Hi Christy would you mind sharing your SL project file here? 

In the meantime here is the workaround:

Create three T/F variables and set the default value to False. On the last slide of each scene, add a trigger to change the variable value to True either when timeline starts or when user clicks a button. So when user reaches/clicks on a button on the last slide of each scene, the variable value changes to true.

Initially set the Next button to either disabled/hidden when timeline starts if the three variables are false. Then add a trigger on desired slides to change the state of Next button to normal when timeline starts once all the three variables are True. Add these two trigger on the slide master so that you don't need to add on each slide. 

Let me know if you need any further help!!