<div class="l-wrap">
  <div class="fixed-gutter-grid">
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
  </div>
</div>

@mixin clearfix {
  &:after {
    display: table;
    clear: both;
    content:'';
  }
}

body {
  margin-top: 3rem;
}

.l-wrap {
  max-width: 1140px;
  margin-right: auto;
  margin-left: auto;
}

.three-col-grid {
  @include clearfix;
}

.grid-item {
  width: calc((100% - 24px * 3) / 3);
  margin-top: 24px;
  padding-left: 12px;
  padding-right: 12px;
  float: left;
}

/* Demo purposes */
.grid-inner {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 80px;
  text-align: center;
  background: rgba(red, 0.25);
}

.column {
  width: calc((100% - 24px * 12) / 12);
  margin-right: 12px;
  margin-left: 12px;
  background: rgba(blue,0.25);
  float:left;
  height: 80px;
}