Currently I am not skilled enought to understand why the following won't work.
Could you please explain? Thanks.

I guess I'll download from level3. Thus we don't want anything from level1&2.

If you want to download everything from level3:
-*mywebsite.com/*
+*mywebsite.com/*/*/*

If you want to download everything from level3 including zip & pdf from
level1&2:
-*mywebsite.com/*
+*mywebsite.com/*.pdf
+*mywebsite.com/*.zip
+*mywebsite.com/*/*/*