Generated by SassMeister.com.

SassMeister-input-HTML.html

<article class="block">1</article>
<article class="block">2</article>
<article class="block">3</article>
<article class="block">4</article>
<article class="block">5</article>
<article class="block">6</article>
<article class="block">7</article>

SassMeister-input.scss

// ----
// Sass (v3.4.9)
// Compass (v1.0.1)
// Susy (v2.1.3)
// ----

@import 'susy';

$susy: (
  global-box-sizing: border-box,
  use-custom: (
    rem: true
  ),
  columns: 12,
  column-width: 4em,
  gutters: 1/4
);

.block {
  @include span(4 of 12);
  
  &:nth-child(3n) {
    @include last;
  }
}

// so you can see it...
.block {
  background: lightblue;
  margin-bottom: 1em;
}

SassMeister-output.css

.block {
  width: 32.20339%;
  float: left;
  margin-right: 1.69492%;
}
.block:nth-child(3n) {
  float: right;
  margin-right: 0;
}

.block {
  background: lightblue;
  margin-bottom: 1em;
}

SassMeister-rendered.html

<article class="block">1</article>
<article class="block">2</article>
<article class="block">3</article>
<article class="block">4</article>
<article class="block">5</article>
<article class="block">6</article>
<article class="block">7</article>