I thought this would be easier than it would.
I have the top level navigation listed horizontally on the top of the page. These don't apply to my problem.
I have a second level navigation in the sidebar however with second-level pages (child pages) and then third-level under those (grandchildren pages).
Problem: I only want the third-level pages shown if you're currently in the second-level page.
Click on the C page and you get:
A B C thirdlevel1 thirdlevel2 thirdlevel3 D
Or Click on B page:
A B thirdlevel1 thirdlevel2 C D