Generated by SassMeister.com.

SassMeister-input-HTML.html

<button class="button--blue--large--rounded">I'm a button!</button>

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

/* Multiple Modifier */
@mixin mm($modifier) {
    // Gets string length
    $len: str-length(#{&});
    $parent: str-slice(#{&}, 2, $len);

    @at-root [class^="#{$parent}"][class*="--#{$modifier}"] {
        @extend .#{$parent};
        @content;
    }
}

.button {
    padding: 1em 2em;
   
    @include mm(blue) {
        background: #00f;
    }
   
    @include mm(large) {
        font-size: 36px;
    }
   
    @include mm(rounded) {
        border-radius: 10px;
    }
}

SassMeister-output.css

/* Multiple Modifier */
.button, [class^="button"][class*="--blue"], [class^="button"][class*="--large"], [class^="button"][class*="--rounded"] {
  padding: 1em 2em;
}
[class^="button"][class*="--blue"] {
  background: #00f;
}
[class^="button"][class*="--large"] {
  font-size: 36px;
}
[class^="button"][class*="--rounded"] {
  border-radius: 10px;
}

SassMeister-rendered.html

<button class="button--blue--large--rounded">I'm a button!</button>