Responsive 3-column preview card component with HTML and CSS

It was an easy one. I tried out grid on a website for the first time and it is awesome to use. I just need feedback on the mobile design; in the media query for mobile devices, I changed the container height to 96vh so as to provide some gap at the top and bottom and used margin : auto auto ; to center it both horizontally and vertically; of course it was not centered vertically so I need suggestions in this area, or if this method of providing it some gap at the top and the bottom for mobile devices by changing its height is absurd of me, do suggest a better method to do this.... Also I provided it rounded borders in a rather long method so any suggestions in this area would be helpful.