Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="ad item">AD</div>
<div class="flex-container">
  <div class="item">item</div>
  <div class="item">item</div>
  <div class="item">item</div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.1)
// Susy (v2.1.3)
// ----

@import "susy";

$susy: (
  columns: 4,
  column-width: 300px,
  gutter-position: inside,
);

.ad {
  @include span(last 1 of 4 static);
}
    
.flex-container {
  @include post(1 of 4 static);
    
  .item {
    @include span(1 of 3);
  }
}

// style
.item {
  background: lightgray;
  height: 200px;
  padding-top: 1em;
  outline: 1px solid;
}

SassMeister-output.css

.ad {
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  width: 375px;
  float: right;
  padding-left: 37.5px;
  padding-right: 37.5px;
}

.flex-container {
  margin-right: 375px;
}
.flex-container .item {
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  width: 33.33333%;
  float: left;
  padding-left: 3.33333%;
  padding-right: 3.33333%;
}

.item {
  background: lightgray;
  height: 200px;
  padding-top: 1em;
  outline: 1px solid;
}

SassMeister-rendered.html

<div class="ad item">AD</div>
<div class="flex-container">
  <div class="item">item</div>
  <div class="item">item</div>
  <div class="item">item</div>
</div>