Panels region only supports one bootstrap class

After changing a Region style to Bootstrap, I am prompted to choose a Column Type and a Column Size. If I select "Large Devices" with a size of "4" the class of "col-lg-4" is applied to that region. But of course, I also want to set a class of "col-md-4" & "col-sm-6" to the same region to accommodate for different screen sizes. Am I missing something obvious because I can't figure this out and it kind of defeats the purpose of these Bootstrap classes if I can only apply one. Thanks.