I have conditional activities already set yes. I guess I must set the proper formula, but have no clue what formula would be best
Here is a link to the calculations help page.
I would suggest using the min function - if you set the total for the course to the minimum score of those three quizzes, if any was less than 80, then they would fail.
I think that is how that works though, to be honest, I have not worked with calculations much.
I was thinking that with conditional activities and completion set up, you could set up the course where it was not completed until they had scored 80 on all three course, but, you are right, you probably need the grade book working correctly too.