When you want to have the last element fluid then you can use float:none;. 
The last element gets the float:none; and also a padding-left equal to the width of the other two fixed divs.
HTML
<div class="fixed first">
    fixed first
</div>
<div class="fixed second">
    fixed second
</div>
<div class="fluid third">
    fluid third
</div>
CSS
.fixed {float:left; }
.first {width:100px; height:100px; background-color:#ccc;}
.second {width:100px; height:100px; background-color:#ddd;}
.fluid { float:none;  padding-left:200px; background-color:#eee; height:100px;}
Here is a demo
Update based on comments (changes only in CSS)
.fixed {float:left; }
.first {width:100px; height:100px; background-color:#ccc; margin-right:10px;}
.second {width:100px; height:100px; background-color:#ddd; margin-right:10px;}
.fluid { float:none;  margin-left:220px; background-color:#eee; height:100px;}
This concept can also wotk with margin instead of padding. So you can do the following. I have added some margin to implement the space between the divs. The margin that the fixed divs should be added to the margin-left of the fluid div.
Here is the updated demo in a jsfiddle. 
In your codepen you just need to add .fluid { margin-left: 220px; } as you can see here.