<===> input.scss
$items: 3;
li {
  &:nth-child(#{$items}n - #{$items}) {
    color: red;
  }
}
<===> output.css
li:nth-child(3n - 3) {
  color: red;
}

<===> output-dart-sass.css
li:nth-child(3n-3) {
  color: red;
}