Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="container">
  <div class="fpo"></div>
  <div class="fpo"></div>
  <div class="fpo"></div>
</div>

SassMeister-input.scss

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

@mixin fpo() {
  background-color: slategray;
  position: relative;

  &:before {
    content: '';
    display: block;
    padding-bottom: 100%;
  }
  
  &:after {
    content: 'FPO';
    color: #fff;
    font: 20px/1 Helvetica;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }
}

.fpo {
  @include fpo();
  width: 20%;
}

.container {
  display: flex;
  justify-content: space-around;
  width: 100%;
}

SassMeister-output.css

.fpo {
  background-color: slategray;
  position: relative;
  width: 20%;
}

.fpo:before {
  content: '';
  display: block;
  padding-bottom: 100%;
}

.fpo:after {
  content: 'FPO';
  color: #fff;
  font: 20px/1 Helvetica;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

.container {
  display: flex;
  justify-content: space-around;
  width: 100%;
}

SassMeister-rendered.html

<div class="container">
  <div class="fpo"></div>
  <div class="fpo"></div>
  <div class="fpo"></div>
</div>