Generated by SassMeister.com.

SassMeister-input-HTML.html

<h1>Heading 1</h1>
<h2>Heading 2</h2>
<h3>Heading 3</h3>

SassMeister-input.scss

// ----
// Sass (v3.3.10)
// Compass (v1.0.0.alpha.20)
// ----

.modal--heading, %modal--heading {
  margin: 5px 0 10px;
  white-space: normal;
  background: red;
  
  &---two {
    font-size: 1.5rem;
    line-height: 1.5rem;
    text-align: left;
    background: green;
  }

  &---three {
    font-size: 1rem;
    line-height: 1rem;
    text-align: center;
    background: blue;
  }
}

h1 {
  @extend %modal--heading;
}

h2 {
  @extend %modal--heading---two;
}

h3 {
  @extend %modal--heading---three;
}

SassMeister-output.css

.modal--heading, h1 {
  margin: 5px 0 10px;
  white-space: normal;
  background: red;
}
.modal--heading---two, h2 {
  font-size: 1.5rem;
  line-height: 1.5rem;
  text-align: left;
  background: green;
}
.modal--heading---three, h3 {
  font-size: 1rem;
  line-height: 1rem;
  text-align: center;
  background: blue;
}

SassMeister-rendered.html

<h1>Heading 1</h1>
<h2>Heading 2</h2>
<h3>Heading 3</h3>