Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="container">
  <div class="col">
    <div class="content">
      Col 1
    </div>
  </div>
  <div class="col">
    <div class="content">
      Col 2
    </div>
  </div>
  <div class="col">
    <div class="content">
      Col 3
    </div>
  </div>
</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@import "susy";

$susy: (
  columns: 12,
  gutters: .01,
  // output: isolate,
  // gutter-position: before,
  container-position: center,
  container: 1200px
);

.container {
  @include container;
  .col {
    @include gallery(4);
    text-align: center;
    background: #333;
    .content {
      padding: 10px;
      background: purple;
    }
  }
}

SassMeister-output.css

.container {
  max-width: 1200px;
  margin-left: auto;
  margin-right: auto;
}

.container:after {
  content: " ";
  display: block;
  clear: both;
}

.container .col {
  width: 33.27828%;
  float: left;
  text-align: center;
  background: #333;
}

.container .col:nth-child(3n + 1) {
  margin-left: 0;
  margin-right: -100%;
  clear: both;
  margin-left: 0;
}

.container .col:nth-child(3n + 2) {
  margin-left: 33.36086%;
  margin-right: -100%;
  clear: none;
}

.container .col:nth-child(3n + 3) {
  margin-left: 66.72172%;
  margin-right: -100%;
  clear: none;
}

.container .col .content {
  padding: 10px;
  background: purple;
}

SassMeister-rendered.html

<div class="container">
  <div class="col">
    <div class="content">
      Col 1
    </div>
  </div>
  <div class="col">
    <div class="content">
      Col 2
    </div>
  </div>
  <div class="col">
    <div class="content">
      Col 3
    </div>
  </div>
</div>