Looping sass and incrementing by 3

SassMeister-input.scss

// ----
// Sass (v3.4.21)
// Compass (v1.0.3)
// ----

.box {
  @for $i from 0 through 9 {
    $c : $i + 1;
    &:nth-child(#{$c}) {
      order: (($i - $i % 3) / 3) + 1;  
    }
  }  
}

SassMeister-output.css

.box:nth-child(1) { order: 1; }
.box:nth-child(2) { order: 1; }
.box:nth-child(3) { order: 1; }
.box:nth-child(4) { order: 2; }
.box:nth-child(5) { order: 2; }
.box:nth-child(6) { order: 2; }
.box:nth-child(7) { order: 3; }
.box:nth-child(8) { order: 3; }
.box:nth-child(9) { order: 3; }
.box:nth-child(10) { order: 4; }