calc demo

width - fixed position percentage

HTML

<div class="box1"></div>
<div class="box2"></div>
<div class="box3"></div>
<div class="box4"></div>
<div class="box4"></div>

CSS

body
{
    margin: 0;
    padding: 0;
}

.box1
{
    width: 50px;
    height: 100px;
    background: red;
    float: left;
}

.box2
{
    width: calc((100% - 50px) / 3);
    height: 100px;
    background: yellow;
    float: left;
}

.box3
{
    width: calc((100% - 50px) / 3);
    height: 100px;
    background: blue;
    float: left;
}

.box4
{
    width: calc((100% - 50px) / 3);
    height: 100px;
    background: green;
    float: left;
}