Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="body">
<div class="portfolio-item">thing</div>
<div class="portfolio-item">thing</div>
<div class="portfolio-item">thing</div>
</div>

SassMeister-input.scss

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

@import "susy";


$susy: (
  columns: 3,
  gutters: 30px/360px,
  column-width: 360px,
);

.body {
  @include container;
  overflow: hidden;
}

.portfolio-item{
    @include gallery(1 of 3);
    background: skyblue;
  }

SassMeister-output.css

.body {
  max-width: 1140px;
  margin-left: auto;
  margin-right: auto;
  overflow: hidden;
}

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

.portfolio-item {
  width: 31.57895%;
  float: left;
  background: skyblue;
}

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

.portfolio-item:nth-child(3n + 2) {
  margin-left: 34.21053%;
  margin-right: -100%;
  clear: none;
}

.portfolio-item:nth-child(3n + 3) {
  margin-left: 68.42105%;
  margin-right: -100%;
  clear: none;
}

SassMeister-rendered.html

<div class="body">
<div class="portfolio-item">thing</div>
<div class="portfolio-item">thing</div>
<div class="portfolio-item">thing</div>
</div>