@function sides($sides) {
  @if type-of($sides) != list {
    $sides: ($sides);
  }

  @if length($sides) == 1 {
    $sides: append($sides, nth($sides, 1));
  }

  @if length($sides) == 2 {
    $sides: append($sides, nth($sides, 1));
  }

  @if length($sides) == 3 {
    $sides: append($sides, nth($sides, 2));
  }

  @return $sides;
}
