You can make a big arrow using :before or :after and create a square that is absolutely positioned to the top of the element (obviously you will need to change the left position for each of the three boxes). Then you just transform:rotate(45deg) the pseudo element to get the square to look like an arrow. If you overlap at the mid position it will look like an arrow.
I didn’t offer code because your fiddle doesn’t look anything like your design (e.g. I see two boxes in a line and a third underneath).