Generated by SassMeister.com, the Sass playground.

SassMeister-input.scss

// beginning of a grid-column sizing mixin
// requires compass

@mixin g-cell($n, $d, $args...) {
    width: $n / $d * 100%;
    // @if length($args) >= 1 { position: relative; }
    @each $arg in $args {
        $dir: nth($arg,1); $p: nth($arg,2);
        @if $p == outside {
            margin-#{opposite-position($dir)}: 100%;    
            margin-#{$dir}: $n / $d * -100%;
            margin-bottom: -100%;
        }
        @else { margin-#{$dir}: $p / $d*100%; }
    }
}


.test {
    @include g-cell(1, 3, left outside);    
}

.test2 {
    @include g-cell(2, 3, right 2, left -1);
}

// @include resize-col(mobile, span 2 3, right 1, left -3);
// @include resize-col(tablet, span 1 3, right 2);
// @include resize-col(desktop, span 3 3, right outside);

// .desktop-span-6-3 
// .desktop-right-plus-1-3
// .desktop-left-minus-2-3
// .desktop-right-outside

SassMeister-output.css

.test {
  width: 33.33333%;
  margin-right: 100%;
  margin-left: -33.33333%;
  margin-bottom: -100%;
}

.test2 {
  width: 66.66667%;
  margin-right: 66.66667%;
  margin-left: -33.33333%;
}