How to make a grid with indents on flexes?

Question

Flexbox

Just a 3xAnyvalue grid
The problem is how to set padding without gap (It seems to have poor support), on grids I would use grid-gap, on flexes I used to do this https://codepen.io/poylar/pen/yLPVdWE + played with nth-child, but I still don't understand the formula for equal spacing, without extra padding from the bottom of the bottommost elements and to the right of the rightmost elements (or left)