CSS: Placing divs left/center/right inside header

Utilize the Magic of Overflow: Hidden

If you can swap the html position of 2 & 3 like so:

<div id="header-e1">
    1 is wider
</div>
<div id="header-e3">
    3 is also
</div>
<div id="header-e2">
    2 conforms
</div>

Then you can set this css which will cause 2 to "fill" the available space because of the overlow: hidden on it. So if 1 & 3 expand, 2 narrows (shrink window down to see what happens at really small size).

#header-e1 {float: left;}
#header-e2 {overflow: hidden;}
#header-e3 {float: right;}

Technically, you could keep your current html order and your float: left on both 1 & 2 and make 3 the flex div with overflow: hidden. You could do the same with 1 by reversing the order of the html completely and setting 2 & 3 to float: right with 1 having overflow: hidden. To me it would seem best to have the middle flex, but you know your application better than I.

If you are trying to make the site with a responsive width, you can try the following (33% is roughly one-third):

#header-e1 {
    float: left;
    width:33%;
    border: 1px solid black;
}

#header-e2 {
    float: left;
    width:33%;
    border: 1px solid black;
}

#header-e3 {
    float: left;
    width:33%;
    border: 1px solid black;
}

You could also used fixed widths for the divs. If you want the further from each other you can play with their left/right margins etc. Hope that helps!

Here is an edit for no widths:

#wrapper {
    position:relative; (add to wrapper)
}

#header-e1 {
    position:absolute;
    left:0;
    border:1px solid black;
}

#header-e2 {
    position:absolute;
    left:50%;
    border:1px solid black;
}

#header-e3 {
    position:absolute;
    right:0;
    border: 1px solid black;
}