Generated by SassMeister.com.

SassMeister-input-HTML.html

<main>
  <div class="third"></div>
  <div class="third"></div>
  <div class="third"></div>
</main>

SassMeister-input.scss

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// Susy (v2.1.2)
// ----

@import "susy";

$susy: (
  columns: 3,
  debug: (
    image: show,
  ),
);

main {
  // Create a 800px container
  @include container(800px);
  padding-top: 20px;

  div {
    background: Tomato;
    height: 50px;
    margin-bottom: 35px;
  }
  
  // Creates a layout of 12 columns with 4 column gutters
  .third {
    @include span(1);
    &:nth-child(3n) {
      @include last;
    }
  }
}

SassMeister-output.css

main {
  max-width: 800px;
  margin-left: auto;
  margin-right: auto;
  background-image: linear-gradient(to right, rgba(102, 102, 255, 0.25), rgba(179, 179, 255, 0.25) 80%, transparent 80%);
  background-size: 35.71429%;
  background-origin: content-box;
  background-clip: content-box;
  background-position: left top;
  padding-top: 20px;
}
main:after {
  content: " ";
  display: block;
  clear: both;
}
main div {
  background: Tomato;
  height: 50px;
  margin-bottom: 35px;
}
main .third {
  width: 28.57143%;
  float: left;
  margin-right: 7.14286%;
}
main .third:nth-child(3n) {
  float: right;
  margin-right: 0;
}

SassMeister-rendered.html

<main>
  <div class="third"></div>
  <div class="third"></div>
  <div class="third"></div>
</main>