HTML, CSS, CSS Flexbox, Button Generator

I recently learned the basics of flexbox, so I used flexbbox extensively. I am still not very confident with flexbox but I worked my way out. I used flexbox for the 3 boxes and the content within the three boxes. I am not sure, but I believe that maybe for the content I should have used grid. I have not been able to align the three cards in the center of the screen. I used a work around by adding a margin to the body of 5%. But that is a very crude way to go about this. I used media query for the first time. I gave the screen size of 375 px for the mobile view. Please let me know, any glaring errors I have done. Other than that, please do let me know the CSS attributes or elements I should have used to improve the code. Thanks!