Grandchild menu clipped by right side of page

I am new to WordPress and am trying to learn it as well as HTML and CSS. I apologize if my post here is poor quality. After a lot of searching I found the Adventurous theme and really like it. So far it meets all my needs except with what I believe are called grandchild or 3rd level menu items. If you look at my staging site you will see a top level menu item "People Groups", under it there are two child or 2nd level menu items: Pueblo and Tepehuan. If you hover over either of the 2nd level menu items, there are two child or 3rd level menu items for each of them. The problem is that if the browser window is set to a medium size, the right side of the 3rd level menus are blocked by the right side of the browser. Is there a way to make these 3rd level menu items open to the left instead of right?