The code that makes this a two-column layout is centered on the $st_cat_counter variable. This is incremented with each iteration and when it’s an even number ($st_cat_counter / 2) a row is closed and a new row is started:
echo '</div><div class="row">';} elseif ($st_cat_counter == 1)
You should simply be able to change “/ 2” to “/ 3”. This changes the break point from every 2nd item to every third item. CSS changes and other edits will be likely to finish the job.