Variables and Conditions

I have a course that is mainly linear but contains a few pages that will show in the menu but aren't a part of the linear navigation. Along the way the user will be prompted to view those pages on their own but we have to track that they have done so. Before they can move to a specific part of the course they will have had to at least opened each one of these pages before they can move forward. 

How do I set up the variables so we can track that they have opened those pages AND how do we show just the onese they have not seen when they get to the IF/THEN part of the course. 

I've attached a down-n-dirty illustration of what we are trying to accomplish. We haven't started building the course yet. We're making sure we can accomplish this first in case we have to redesign the scenario before building in Storyline. 

In the scenario illustrated the user has visited page A but not B and C so when they get to the IF/THEN part of the course the page they see will only show them links to the two pages they haven't visited yet. If they had visited all three of them then they would be able to continue. 

TIA.