@mixin rounded($vert, $horz, $radius: 10px) {
  border-#{$vert}-#{$horz}-radius: $radius;
  -moz-border-radius-#{$vert}#{$horz}: $radius;
  -webkit-border-#{$vert}-#{$horz}-radius: $radius;
}

div { background-color: #ABC; width: 200px; height: 100px; margin: 20px; }
.div1 { @include rounded(top, left); }
.div2 { @include rounded(top, left, 20px); }
.div3 { @include rounded(top, left, 30px); }