[connorjburton]

Honestly, I just use

  .something
  {
  width: calc(33.33% - #{$spacing-small*3/2});
  margin-left: $spacing-small;

    &:first-of-type
    {
    margin-left: 0;
    }
  }

Well simple

[connorjburton]

Correction:

  .something
  {
  width: calc(33.33% - #{$spacing-small*2/3});
  margin-left: $spacing-small;

    &:first-of-type
    {
    margin-left: 0;
    }
  }

[err4nt]

  .something {
    width: calc(33.33% - .66em);
    margin-left: 1em;
  }
  .something:first-of-type {
    margin-left: 0;
  }

[qbert]

For the gutters, just do:

  .something {
    width: calc(33.33% - #{$spacing-small * 2/3});

    + .something {
      margin-left: $spacing-small;
    }
  }

[connorjburton]

I like that!