Examples:


I suppose you could get away with stacking the existing icons. I might give that a shot to see how it looks.
I think there is a bad link there for one of thoughs here is another example: 
+1
+1
+1
+1
+1 (two squares is better than three, IMO, especially at smaller sizes)
+1
+1
+1
+1
+1
Personally I use this snippet to produce "select all checkbox" :
鈿狅笍 May use pro features
<div style="height: 1em; width: 1em;">
<i class="far fa-square" style="position: absolute; top: 0px; left: 0px;"/>
<i class="fas fa-square fa-inverse" style="position: absolute; top: 2px; left: 2px;"/>
<i class="far fa-check-square" style="position: absolute; top: 3px; left: 3px;"/>
<!--This line for "unselect all" icon
<i class="far fa-square" style="position: absolute; top: 3px; left: 3px;"/>
-->
</div>

Thanks man for provided solution.
JSX Bootstrap v4.0
JSX
<div className="selectAllIcons" onClick={() => this.addAllGroups()} title="Wybierz wszystko">
<i class="fa fa-square" style={{position: "absolute", top: "0px", left: "0px"}}/>
<i class="fa fa-square fa-inverse" style={{position: "absolute", top: "2px", left: "2px"}}/>
<i class="fa fa-check-square" style={{position: "absolute", top: "3px", left: "3px"}}/>
</div>
<div className="unSelectAllIcons" onClick={() => this.generatingGroups()} title="Odznacz wszystko">
<i class="fa fa-square" style={{position: "absolute", top: "0px", left: "0px"}}/>
<i class="fa fa-square fa-inverse" style={{position: "absolute", top: "2px", left: "2px"}}/>
<i class="fa fa-check-square" style={{position: "absolute", top: "3px", left: "3px"}}/>
<i class="fa fa-square" style={{position: "absolute", top: "3px", left: "3px"}}/>
</div>
.unSelectAllIcons,
.selectAllIcons {
height: 1em;
width: 1em;
position: relative;
display: inline-block;
margin: 0 20px 0 20px;
}
PREVIEW

Most helpful comment
Personally I use this snippet to produce "select all checkbox" :
鈿狅笍 May use pro features