Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----

/// Create a compound selector using `:nth-child` to target 
/// children that are multiples of `$n`
///
/// @param {Number} $n
///
/// @example scss - Multiples of 3
///   li {
///     @include nth-child-of(3) {
///       background: red;
///     }
///   }
/// @example css - Result
///   li:nth-child(6n + 1), li:nth-child(6n + 2), li:nth-child(6n + 3) {
///     background: red;
///   }
@mixin nth-child-of($n) {
  $selector: ();
  
  @for $i from 1 through $n {
    $selector: append($selector, '&:nth-child(#{$n * 2n} + #{$i})', 'comma');
  }
  
  #{$selector} {
    @content;
  }
}

li {
  @include nth-child-of(3) {
    background: red;
  }
}

/// Create a unique selector using `:nth-child` and `:not` to target
/// children that are multiples of `$n`
///
/// @param {Number} $n
/// @param {Number} $initial [$n] - Internal only, don't touch
///
/// @example scss - Multiples of 3
///   li {
///     @include r-nth-child-of(3) {
///       background: red;
///     }
///   }
/// @example css - Result
///   li:not(:nth-child(6n + 3)):not(:nth-child(6n + 2)):not(:nth-child(6n + 1)) {
///     background: red;
///   }
@mixin r-nth-child-of($n, $initial: $n) {
  @if $n == 0 {
    @content;
  } @else {
    &:not(:nth-child(#{$initial * 2n} + #{$n})) {
      @include r-nth-child-of($n - 1, $initial) {
        @content;
      }
    }
  }
}

li {
  @include r-nth-child-of(3) {
    background: red;
  }
}

SassMeister-output.css

li:nth-child(6n + 1), li:nth-child(6n + 2), li:nth-child(6n + 3) {
  background: red;
}

li:not(:nth-child(6n + 3)):not(:nth-child(6n + 2)):not(:nth-child(6n + 1)) {
  background: red;
}