3-column preview card component using CSS Flexbox

After coding along to tutorials for about a month I realized I was just coding along and not grasping concepts. So, I decided to give this a shot. It was pretty challenging but I learnt a lot!

Here are some challenges I encountered:

• Responsiveness: I used a desktop first approach to create the card. I then worked on the responsiveness but I wasn't able to make it responsive across all devices. So, I opted for a column layout for devices 768px and below. But, I feel this wasn't the best approach. Recommendations on how I could have done this better will be appreciated.
• Using CSS classes: I'm still learning how to use classes appropriately. I feel like I could have written less CSS than I did. Any feedback on how I've used CSS classes will be appreciated.
• Semantics: Finally, I'll appreciate any feedback on HTML semantics.

Finally, any additional comments and recommendations are welcome. Thanks! :-)