There are no options for styling archive pages (I always though that was a missing thing from @weaver theme 🙂 )
But we can do some with Custom CSS.
If it is just to show the posts in 3 columns on Archive page, you can extrapolate the information I gave in in Item #3 of the thread below
If you want 3 columns on desktop and 2 on mobile you can add the rules below in Main Options > Fonts & Custom > Custom CSS Rule box
.is-desktop.archive article {float:left;width:32.66%;margin-right:1%;clear:none;}
.is-desktop.archive article:nth-of-type(3n) {margin-right:0;}
.is-desktop.archive article:nth-of-type(3n+1) {clear:both;}
.is-desktop.archive article:nth-of-type(3n) {margin-right:0;}
.is-desktop.archive article:nth-of-type(3n+1) {clear:both;}
.is-mobile.archive article {float:left;width:49.5%;margin-right:1%;clear:none;}
.is-mobile.archive article:nth-of-type(2n) {margin-right:0;}
.is-mobile.archive article:nth-of-type(2n+1) {clear:both;}
If not, or if it needs tuning, give us a link to your site both blog page and archive page and explain what you want to do.