There is a cleaner way of doing this for responsive skins. In the base scss file there are these lines
// Small layouts
@include masonry(phone-min, 1);
// Medium layouts
@include masonry(phone-to-tablet, 2);
// Large layouts
@include masonry(tablet-to-desktop, 3);
// X-large layouts
@include masonry(desktop-and-bigger, 4);
to change to a 3 column layout for desktop (X-large) you only need to add this line to your style.scss file
// X-large layouts
@include masonry(desktop-and-bigger, 3);
and you are good to go!