I'm not sure how bs3 = col-sm-6 col-md-4.... applies to the Masonry template. There isn't anything on the template that refers to those types of options. You choose image and thumbnail sizes and whether or not the do max fit, shrink, stretch, etc.
What I ended up having to do with the Masonry template is not use thumbnails. If I tell it to output just the Image and then restrict it to a size that still fits on phones then it is somewhat responsive.