Gallery shortcode defaults

I have tweaked the gallery shortcode to add support 3 different column layouts (bootstrap).
Like this
[gallery ids="..." small="6" medium="4" large="3"]

I have tried to define defaults for each size (line 40-42) but they don’t work.

Preferably i would like to add the three sizes with default so they are placed in the editor. Like the way the id’d are automatically placed when you add the gallery.

Any help on this would be great!