Generated by SassMeister.com.

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SassMeister-input-HTML.html

<div class="container">
  <div class="box">1</div>
  <div class="box">2</div>
  <div class="box">3</div>
  <div class="box">4</div>
  <div class="box">5</div>
  <div class="box">6</div>
  <div class="box">7</div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.4.21)
// Compass (v1.0.3)
// ----

.container {
  display: grid;
  grid-template-columns: repeat(6, 1fr);
  grid-gap: 10px;
  > * {
    &:not(:nth-child(n+3)) {
      grid-column: span 3;
      color: red;
    }
    &:nth-child(n+3) {
      grid-column: span 2;
      color: blue;
    }
  }
}
.box {
  min-width: 200px;
  min-height: 100px;
  background: lightgray;
  font-size: 2em;
  display: flex;
  align-items: center;
  justify-content: center;
}

SassMeister-output.css

.container {
  display: grid;
  grid-template-columns: repeat(6, 1fr);
  grid-gap: 10px;
}
.container > *:not(:nth-child(n+3)) {
  grid-column: span 3;
  color: red;
}
.container > *:nth-child(n+3) {
  grid-column: span 2;
  color: blue;
}

.box {
  min-width: 200px;
  min-height: 100px;
  background: lightgray;
  font-size: 2em;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-align: center;
  -webkit-align-items: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
      -ms-flex-pack: center;
          justify-content: center;
}

SassMeister-rendered.html

<div class="container">
  <div class="box">1</div>
  <div class="box">2</div>
  <div class="box">3</div>
  <div class="box">4</div>
  <div class="box">5</div>
  <div class="box">6</div>
  <div class="box">7</div>
</div>