3 Cards Projects With FlexBoxes

It's my first project and I really need help. So i managed to make everything looks good and adapt both to computer and mobile sizes. But i just don't know how to make the corners rounded. I noticed that even if i set correctly my border-radius property, the #content(which is the container of the 3cards and a flexbox) element's have some empty space on the right and left side, which makes the corners invisible. I don't if it's clear but please help me ! And also feel free to make other suggestions !