Using placeholder to show how it can be used to better effect than extend in most cases

placeholder-extend.css

.class1, .class2, .class3 {
  font-size: 2em;
}

.class3 {
  color: green;
}

placeholder-extend.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.7)
// ----

%extend1{
    font-size: 2em;
}
%extend2{
    color: green;
}
%extend3{
    background: orange;
}

.class1{
    @extend %extend1;
}
.class2{
    @extend %extend1;
}
.class3{
    @extend %extend1;
    @extend %extend2;
}