How do I divide a page into even rows for a comic layout?

Hello everyone.  I'd like to have 3 rows of panels for a comic book.  I've figured out how to align my page into colums, but not rows.  Must I insert a giant table in order to have even row lines on my page?  Using the "preferences-->gridlines" I am only able to set fixed distance gridlines (i.e. 2 inches apart) rather than proportional gridlines (i.e. divide the page into thirds).

What is the easiest way to display gridlines on my page that are in perfect thirds, and also inside my margins?  Thanks so much!