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.
EG.
A
B
C
D
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