The major subject areas have a page under "Subjects." Once you click on a subject, you'll be brought to a page of information and resources for that subject.
Scroll down to the bottom of the pages to see the "Subpages" for each subject.
Also, 3rd grade has created one site with recommended online sites, which have been shared already. https://sites.google.com/cusdk8.org/third-grade-resources/home