SassMeister-input-HTML.jade

div
  - for (var i = 0; i < 10; ++i)
    div.item

SassMeister-input.scss

// ----
// Sass (v3.4.4)
// Compass (v1.0.1)
// Breakpoint (v2.5.0)
// Singularity.gs (v1.4.0)
// ----

@import "breakpoint";
@import "singularitygs";
@include add-grid(3);
@include add-grid(5 at 500px);
@include add-gutter(1/3);
@include sgs-change('output', 'float');

html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}
.item {
  background-color: blue;
  height: 100px;
  @include float-span(1);
  border: 2px solid black;
  margin-bottom: 20px;
  &:nth-child(3n) {
    @include float-span(1, last);
  }
}

SassMeister-output.css

html {
  box-sizing: border-box;
}

*, *:before, *:after {
  box-sizing: inherit;
}

.item {
  background-color: blue;
  height: 100px;
  width: 27.27273%;
  clear: right;
  float: left;
  margin-left: 0;
  margin-right: 9.09091%;
  border: 2px solid black;
  margin-bottom: 20px;
}
.item:nth-child(3n) {
  width: 27.27273%;
  clear: right;
  float: right;
  margin-right: 0;
}

SassMeister-rendered.html

<div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>