I searched high and low in the forums and see no definitive answer.
My guess is that once you have played 3 tournaments within a 7 day period, you have to wait until exactly 7 days (168 hours and no minutes) have passed from time you started the first one before you can enter another.
I have only 3 tournaments per week, and I used them, so how do I know when the next week starts time and have a new 3 tournaments?