Responsive 3 Column Preview Card Using CSS Flexbox

Hi,

I hope this is mostly correct. One issue I am having is that the card heights are different when changing the size of the page. Once the page reaches a certain width, the cards are fine, but until then they seem to be different heights. Is there anything I can please do to change this?

I also think I may have used too many classes, and could have used more specific selectors such as nth of type selectors or descendent selectors. Is the way I have done it easily readable?, or should I adjust it to make it easier to read and change for other people?

Thank you very much, any feedback is definitely welcome.