<div class="l-wrap">
  <div class="l-guest-article">
    <div class="grid-item l-guest"> <div class="grid-inner">Guest profile </div></div>
    <div class="grid-item l-main"><div class="grid-inner">Main article</div></div>
    <div class="grid-item l-sidebar"><div class="grid-inner">Sidebar</div></div>
  </div>
</div>

<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:'';
  }
}


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


body {
  margin-top: 3rem;
}

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

.l-guest-article {
  .l-guest {
    width: percentage(2/12);
    padding-left: 12px;
    padding-right: 12px;
    float: left;
  }

  .l-main {
    width: percentage(7/12);
    padding-right: 12px;
    padding-left: 12px;
    float: left;
  }

  .l-sidebar {
    width: percentage(3/12);
    padding-right: 12px;
    padding-left: 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;
}