Grid layout not aligning properly

I’m trying to make the theme into 3 columns grid layout on large desktop and 2 columns on smaller device such as mobile. I changed the post class to ‘col-xs-6 col-md-4’ in content-grid.php and change ( $count % 2 == 0 ) to ( $count % 3 == 0 ) in index.php.

The images are aligning well on large desktop. However, they are not aligning properly on smaller device. The grid becomes like this:

0 0
0
0 0
0
0 0

You can see it on my website: https://storeimgs.com. What should I do to make them aligning properly?