Responsive 3 Column Layout - Flexbox

This was my first responsive page design task and I feel like it went pretty well, for the majority of tutorials and reading I've done they said that using fixed widths for containers was a bad idea so I feel like I may have committed a cardinal sin at some point?

My main focus was to get flexbox working as intended, however I feel like an additional media query at a tablet-sized screen may have been useful as the design looks poor between 700-900px

As I said this is my first project so feedback is most certainly welcome and I appreciate any time you take looking through my solution.

Cheers, Dan