need help with grid div sizes

[demiurgeQuantified]

so, i'm revamping my website from scratch, and i'm trying to do a thing where the main layout of the site is built with a css grid since it seems rather easy and pretty cool. but i'm having a pretty hard time getting the grid columns the way i want them :(( it's a 3 column grid, and all the tools and resources for column sizing that i can find are based around two column grids. i want them set up in a 20%-60%-20% way but no matter what i try with the grid-template-columns thing none of it changes the sizes!!! i'm getting rly irritated by this so i'd like some help. i'm attaching an image of the wip css below

edit: i'd also like to give the larger container (sitebox) a rounded edge rather than sharp corners, so tips on that would be appreciated