I just installed the theme and the 3 column grid images are not automatically sized to a square. I’ve ensured all plugins are installed and tried switching between masonry and grid and there is no difference.
If you select the “3-column grid” in Theme Options -> Blog page layout, there are no other actions needed for the images to get resized to rectangulars (like here). If this is not happening for you, it could be that you need to regenerate your thumbnails. That can be done with this plugin.