Extend the stack scheme from fontawesome.io to stack three icons at once. The CSS gives the icons different sizes and colours.

three-stacked-fa-icons.css

/* original source: http://fontawesome.io/examples/#stacked */

.fa-stack {
    display: inline-block;
    height: 2em;
    line-height: 2em;
    position: relative;
    vertical-align: middle;
    width: 2em;
}
.fa-stack-1x, .fa-stack-2x, .fa-stack-3x {
    left: 0;
    position: absolute;
    text-align: center;
    width: 100%;
}
.fa-stack-3x {
    color: #0f376d;
    font-size: 2em;
}
.fa-stack-2x {
    color: #2f578d;
    font-size: 1em;
    line-height: inherit;
    top: -0.1em;
}
.combilogo .fa-stack-1x {
    bottom: -0.15em;
    color: #e1f006;
    font-size: 0.5em;
    height: 93%;
    left: auto;
    line-height: inherit;
    right: -0.01em;
    width: 70%;
}

three-stacked-fa-icons.html

<span class="fa-stack fa-4x">
  <span class="fa fa-square-o fa-stack-3x"></span>
  <span class="fa fa-desktop fa-stack-2x"></span>
  <span class="fa fa-sliders fa-stack-1x"></span>
</span>