You need to complete all three exams within a period...

No. The first clause says that you have to take 3 exams in 18 months. The second says you don't.

Thanks.

Suppose you take your first exam in month #1 and take your second exam in month #19. Since you took your second exam outside the defined window of 18 months therefore the result of your first exam is cancelled. Now to complete your course, you need to take your first and third exams between month #19 and month #36.

I hope that you get what I'm trying to say. How to say al

Now I understand! It's actually a simple concept, but I find it hard to explain clearly. Perhaps you need to include an example of the kind you posted.

You must pass three exams within an 18-month period to complete your course. The credit for passing an exam expires after 18 months, and that exam must be retaken.

Suppose George passes his first exam on January 1, 2015;

Thank you.

I think that that will work!

You must pass all three exams within a time window of 18 months. They can be taken in any order.