I know this is a very old question. Just posting this here as I solved this problem using FlexBox. Here is the solution
#container {
height: 100%;
width: 100%;
display: flex;
}
#leftThing {
width: 25%;
background-color: blue;
}
#content {
width: 50%;
background-color: green;
}
#rightThing {
width: 25%;
background-color: yellow;
}
_x000D_
<div id="container">
<div id="leftThing">
Left Side Menu
</div>
<div id="content">
Random Content
</div>
<div id="rightThing">
Right Side Menu
</div>
</div>
_x000D_
Just had to add display:flex
to the container! No floats required.