How have you configured the RG on the Appearance tab of the property editor? If it’s not set to a fixed number of columns, then check the minimum column width.
Then why would you expect it to stack? You just set it to a fixed number of columns.
To get what you’re after, try unchecking that option and then playing with the min column width and max RG width. For example, setting the RG max width to 900 and the min column width to 300 will create 3 columns on large displays and wrap the cells when the RG width drops below 900.
(You’ll want to disable the separator as well - or at least take its width into account when determining the RG max width.)