You can also adjust the masks of your shape in the main comp window. Remember to edit individual points for you mask choose the selection tool and click once on one mask path point. Once you see the point filled in like the image below that means its ready to be edited. If you study the image below you will see only one point is filled in. The other two points are empty. This demonstrates how to edit your mask points:

You can also add points to your masks by selecting the pen tool and hovering over the mask where you want to make the point. You will see a + sign. Click on the mask to make an additional point. After adding a mask point drag this point to the desired location to edit this mask shape:

You are going to add your show information for your lower third object.