Generated by SassMeister.com.

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last 2 versions

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: flex;
  flex-flow: row wrap;
  margin-bottom: -10px;
  margin-right: -10px;
  > * {
    margin-right: 10px;
    margin-bottom: 10px;
    &:not(:nth-child(n+3)) {
      flex-basis: calc(50% - 10px);
      flex-grow: 1;
      color: red;
    }
    &:nth-child(n+3) {
      flex-basis: calc((100%/3) - 10px);
      flex-grow: 0;
      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: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-flex-flow: row wrap;
      -ms-flex-flow: row wrap;
          flex-flow: row wrap;
  margin-bottom: -10px;
  margin-right: -10px;
}
.container > * {
  margin-right: 10px;
  margin-bottom: 10px;
}
.container > *:not(:nth-child(n+3)) {
  -webkit-flex-basis: calc(50% - 10px);
      -ms-flex-preferred-size: calc(50% - 10px);
          flex-basis: calc(50% - 10px);
  -webkit-box-flex: 1;
  -webkit-flex-grow: 1;
      -ms-flex-positive: 1;
          flex-grow: 1;
  color: red;
}
.container > *:nth-child(n+3) {
  -webkit-flex-basis: calc((100%/3) - 10px);
      -ms-flex-preferred-size: calc((100%/3) - 10px);
          flex-basis: calc((100%/3) - 10px);
  -webkit-box-flex: 0;
  -webkit-flex-grow: 0;
      -ms-flex-positive: 0;
          flex-grow: 0;
  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>