Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="list-item">1</div>
<div class="list-item">2</div>
<div class="list-item">3</div>
<div class="list-item">4</div>
<div class="list-item">5</div>
<div class="list-item">6</div>
<div class="list-item">7</div>
<div class="list-item">8</div>
<div class="list-item">9</div>

SassMeister-input.scss

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// ----

@function strip-units($number) {
  @return $number / ($number * 0 + 1);
}

@mixin nth-child($an: 2n, $sibling: "*", $count: 15) {
  $index: "";
  $n: false;
  $selector: "";
  $modifier: 0;
  @if type-of($an) == list {
    $index: nth($an, 1);
    $modifier: nth($an, 2);
  }
  @else {
    $index: $an;
  }
  @if $index == "even" or $index == even {
    $index: 2n;
  }
  @if $index == "odd" or $index == odd {
    $index: 2n;
    $modifier: 1;
  }
  @if unit($index) == "n" {
    $index: strip-units($index);
    $n: true;
  }
  @if $modifier < 0 {
    @if abs($modifier) > $index {
      $modifier: -(abs($modifier) % floor($index));
    }
    $modifier: $modifier + floor($index);
  }
  @if $index > 0 {
    @if $n == false {
      $count: 1;
    }
    @if $index == 1 {
      $selector: "&:first-child";
    }
    @if $modifier > 1 {
      @for $j from 0 to $modifier - 1 {
        $selector: $selector + "+ " + $sibling;
      }
      $selector: $selector + ",";
    }
    @for $i from 1 to $count + 1 {
      @if $i > 1 {
        $selector: $selector + ", ";
      }
      @for $x from 1 - $modifier to $index * $i {
        $selector: $selector + "+ " + $sibling;
      }
    }
  }
  @if $index > 0 {
    @if $modifier == 1 {
      @content;
    }
    #{$selector} {
      @content;
    }
  }
}

.list-item {
  width: 32.33334%;
  margin-right: 1%;
  float: left;
  clear: none;
  box-shadow: inset 0 0 1px red;
}

.list-item {
  @include nth-child($an: 3n, $sibling: "div", $count: 3) {
    margin-right: 0;
  }
  @include nth-child($an: 4n, $sibling: "div", $count: 3) {
    clear: left;
  }
}

SassMeister-output.css

.list-item {
  width: 32.33334%;
  margin-right: 1%;
  float: left;
  clear: none;
  box-shadow: inset 0 0 1px red;
}

.list-item + div + div, .list-item + div + div + div + div + div, .list-item + div + div + div + div + div + div + div + div {
  margin-right: 0;
}
.list-item + div + div + div, .list-item + div + div + div + div + div + div + div, .list-item + div + div + div + div + div + div + div + div + div + div + div {
  clear: left;
}

SassMeister-rendered.html

<div class="list-item">1</div>
<div class="list-item">2</div>
<div class="list-item">3</div>
<div class="list-item">4</div>
<div class="list-item">5</div>
<div class="list-item">6</div>
<div class="list-item">7</div>
<div class="list-item">8</div>
<div class="list-item">9</div>