Make child divs inside a parent div equal width and fluid - html

Question

How can I make the 3 child divwith class .box have the same width while fluidly occupying the entire parent container div while staying inline.
Here is a fiddle
#container {
width: 20em;
background: red;
text-align: center;
}
.box {
display: inline-block;
margin: 1em;
border: 1px solid #000;
}
<div id="container">
<div class="box">test</div>
<div class="box">test</div>
<div class="box">test</div>
</div>

Answer 1

use CSS flexbox for that
#container {
width: 20em;
display: flex;
background: red;
text-align: center;
}
.box {
flex: 1;
border: 1px solid #000;
}
<div id="container">
<div class="box">test</div>
<div class="box">test</div>
<div class="box">test</div>
</div>
OR
use inline-block as you already are using, with a few tweaks.
* {
box-sizing: border-box
}
#container {
width: 20em;
background: red;
text-align:center
}
.box {
font-size: 16px;
display: inline-block;
width: calc(100% / 3);
border: 1px solid #000;
}
<div id="container">
<div class="box">test</div><div class="box">test</div><div class="box">test</div>
</div>
OR
use CSS tables for old browsers support
#container {
width: 20em;
display: table;
background: red;
text-align: center;
}
.box {
display:table-cell;
border: 1px solid #000;
}
<div id="container">
<div class="box">test</div>
<div class="box">test</div>
<div class="box">test</div>
</div>

Answer 2

display:table version:
#container {
width: 20em;
background: red;
text-align: center;
display: table;
table-layout: fixed;
border-spacing: 1em;
/* instead the margin:1em; you applied to children */
}
.box {
display: table-cell;
;
border: 1px solid #000;
}
.middle{vertical-align:middle;}
<div id="container">
<div class="box">test</div>
<div class="box">test</div>
<div class="box">test</div>
</div>
<hr/>
<div id="container">
<div class="box">test
<br/>+ 1line ?</div>
<div class="box">test</div>
<div class="box middle">test</div>
</div>
table-layout:fixed will fix width value you set, for main container and children.
if children(table-cell) have no width set, they will spray evenly
#container {
width: 20em;
background: red;
text-align: center;
display: table;
table-layout: fixed;
border-spacing: 1em;
/* instead the margin:1em; you applied to children */
}
.box {
display: table-cell;
;
border: 1px solid #000;
}
.middle{vertical-align:middle;}
<div id="container">
<div class="box">test test test test </div>
<div class="box">test</div>
<div class="box">test</div>
</div>
<hr/>
<div id="container"class=" bis ">
<div class="box">testtesttesttest testtesttest</div>
<div class="box">test</div>
<div class="box">test</div>
</div>

Answer 3

If you want to do this with display: inline-block you can set equal width of 33.33% on each .box, remove white space from HTML and also add box-sizing: border-box.
* {
box-sizing: border-box;
}
#container {
width: 20em;
background: red;
text-align: center;
}
.box {
display: inline-block;
width: 33.33%;
border: 1px solid #000;
}
<div id="container">
<div class="box">test</div><div class="box">test</div><div class="box">test</div>
</div>