Mobile & Monitor Responsive Card, Column Width

Hey all. I'm new to Bootstrap (have been using it for about a month) and have been using the cards from the Sb-admin-site-2 as a template for a site I'm building. Currently, I am having two card- related problems. I recently spent a ton of time customizing the columns that my cards are in to be inline-flex so that on a computer screen there will be a row of three cards. However, so far I haven't found a way to change the size/width of the cards, only the width of the columns. And secondly, ever since I changed the display style to inline flex, when on a mobile screen, the cards shrink into a 3 column layout instead of becoming larger and into a more elegant vertical layout, with 3 cards stacked on top of each other. My end goal is to have a mobile screen with a layout of three vertical cards, and a computer screen of 3 horizontal cards (all with even spacing & width.) I'm not sure where I am going wrong here, so any advice is appreciated.

Can you post a link to the page so we can look at it?