col-xs-* kicks in when the width of the viewport is greater than 480px, if you want to show three cards per row regardless of the size of the screen estate, you should use
Additionally, there are also
col-sm-* which kicks in when browser is wider than 768px (tablet portrait),
col-md- which kicks in when browser is wider than 992px (small desktop, tablet landscape), and finally
col-lg-* for large desktop monitors which kick in when browser is wider than 1440px.
All these different classes help you build a fluid grid system that appropriately scales up to 12 columns as the device or viewport size resizes.