How to override Bootstrap3 GridFramework?
The Bootstrap3.php file is setting the column classes.
You could make another grid type file called Bootstrap3med.php or Bootstrap3sm.php for example. Copy Bootstrap3.php, and change the class inside from "class Bootstrap3 extends GridFramework" to something like "class Bootstrap3med extends GridFramework".
In your theme, open page_theme.php and change (or add) "protected $pThemeGridFrameworkHandle = 'bootstrap3';" to "protected $pThemeGridFrameworkHandle = 'bootstrap3med';"
Thanks for that option, I have actually just done exactly that as a stop gap, but don't really want to put new classes in the core. Any idea how to use a class from, for example a custom theme directory?