A agree with avoiding dropdowns here... they would be too long.
Looking at the image, it seems the top right horiz menu would be level 2, and the box underneath level 3... because as a user I would expect that if I click on something on that top menu, it will show me the subchildren who belong under that topic in the box below.
If you have an account on github you should check out how they manage multiple levels... they do something very similar, since there are many ways to navigate to (potentially) lots of info. And when I'm on my page, there's a "second level menu" with choices like code, network, etc. if I click on code I see the folders of the repo I'm in in the box below (choosing a repo would be level one). If I click network I see the branches of the repos below. Etc.