Mobile first responsive 3 column preview card made using CSS flexbox

Big black bars on the side! I checked your code and noticed that you give things a fixed width using px. Here's some quick tips:

1. A background is applied to a box, in this case your body element. To make your body cover the full page, try using 'height: 100vh' and 'width: 100%'.
2. Step away from using px, and familiarise yourself with sizing boxes using flex or units like 'rem'. Seriously, google these things before you start your next project; it's pretty simple and it'll improve everything about your code.