3 Fluid Divs width 2 Fixed Margins between them?

Question

I have the following setup, but setting the width of the divs to a something around 30% is not working consistently (once the window width goes less than some number the third divs drops below..

Is there a better way of doing this, so that my divs always stay inline and keep getting smaller and smaller while the margin stays fixed at 18px between them ?

CSS :

.parent {
  width: 100%;
  height: 50px;
}  

.child {
  float: left;
  margin-right: 18px;
  border: 2px solid white;
  text-align: center;
  line-height: 50px;
  height: 100%;
  width: ~30%; /* doesn't work */
}  

.child:nth-child(3) {
  margin-right: 0;
}

HTML :

<div class="parent">
  <div class="child">one</div>
  <div class="child">two</div>
  <div class="child">three</div>
</div>

Answer 1

If you are looking for IE8 support, and can change your markup, you can nest the blocks inside 33.33% width elements.

For IE8 support, you need to get rid of the nth-child() declaration. In order to have inner gaps only, I used the technique described here : Items grid with inner padding only.

DEMO

body{
    overflow:hidden;
    margin:0;
}
.wrap{
    margin: 0 -9px;
}
.box {
    width:33.33%;
    float:left;
}
.box div {
    background:grey;
    height:150px;
    margin:0 9px;
}

<div class="wrap">
    <div class="box">
        <div>one</div>
    </div>
    <div class="box">
        <div>two</div>
    </div>
    <div class="box">
        <div>three</div>
    </div>
</div>

Answer 2

Use calc() to calculate column width.

.child {width: calc(33.333% - 12px);} /* margins in total have 36px / 3 cols  */

Calc is supported since IE9.