Generated by SassMeister.com.

SassMeister-input-HTML.jade

.row
  .item
  .item
  .item

SassMeister-input.scss

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

@import "susy";


.row {
  .item {
    float: left;
    width: 30%;
    background: gray;
    margin-right: 3%;
    min-height: 100px;
    margin-bottom: 20px;
  }
}

@media(max-width: 1200px) {
  .row {
    .item {
      width: 45%;
      margin: 2.5%;
      &:nth-child(3) {
        width: 95%;
        margin: 2.5%;
      }
    }
  }
}

SassMeister-output.css

.row .item {
  float: left;
  width: 30%;
  background: gray;
  margin-right: 3%;
  min-height: 100px;
  margin-bottom: 20px;
}

@media (max-width: 1200px) {
  .row .item {
    width: 45%;
    margin: 2.5%;
  }
  .row .item:nth-child(3) {
    width: 95%;
    margin: 2.5%;
  }
}

SassMeister-rendered.html

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