Just give them a width and float: left;, here’s an example:

<div style="width: 500px;">
 <div style="float: left; width: 200px;">Left Stuff</div>
 <div style="float: left; width: 100px;">Middle Stuff</div>
 <div style="float: left; width: 200px;">Right Stuff</div>
 <br style="clear: left;" />
</div>

The modern way is to use the CSS flexbox, see support tables.

.container {
  display: flex;
}
.container > div {
  flex: 1; /*grow*/
}

<div class="container">
  <div>Left div</div>
  <div>Middle div</div>  
  <div>Right div</div>
</div>

You can also use CSS grid, see support tables.

.container {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr; /* fraction*/
}

<div class="container">
  <div>Left div</div>
  <div>Middle div</div>  
  <div>Right div</div>
</div>

It is same way as you do for the two divs, just float the third one to left or right too.

<style>
  .left{float:left; width:33%;}
</style>

<div class="left">...</div>
<div class="left">...</div>
<div class="left">...</div>

float them all left

make sure a width is specified that they can all fit in their container (either another div or the window), otherwise they will wrap

<br style="clear: left;" />

that code that someone posted up there, it did the trick!!!
when i paste it just before closing the Container DIV, it helps clear all subsequent DIVs from overlapping with the DIVs i’ve created side-by-side at the top!

<div>
<div class="left"></div>
<div class="left"></div>
...
...
<div class="left"></div>
<!--  then magic trick comes here  -->
<br style="clear: left;" />
</div>

tadaa!! 🙂

Float all three divs to the left. Like here:

.first-div {
  width:370px;
  height:150px;
  float:left;
  background-color:pink;
}

.second-div {
  width:370px;
  height:150px;
  float:left;
  background-color:blue;
}

.third-div {
  width:370px;
  height:150px;
  float:left;
  background-color:purple;
}

<style>
.left-column
{
float:left;
width:30%;
background-color:red;
}
.right-column
{
float:right;
width:30%;
background-color:green;
}
.center-column
{
margin:auto;
width:30%;
background-color:blue;
}
</style>

<div id="container">
<section class="left-column">THIS IS COLUMN 1 LEFT</section>
<section class="right-column">THIS IS COLUMN 3 RIGHT</section>
<section class="center-column">THIS IS COLUMN 2 CENTER</section>
</div>

the advantage of this way is you can set each column width independant of the other as long as you keep it under 100%, if you use 3 x 30% the remaining 10% is split as a 5% divider space between the collumns

I usually just float the first to the left, the second to the right. The third automatically aligns between them then.

<div style="float: left;">Column 1</div>
<div style="float: right;">Column 3</div>
<div>Column 2</div>

you can float: left for all of them and set the width to 33.333%

try to add “display: block” to the style

<style>
   .left{
          display: block;
          float:left; 
          width:33%;
    }
</style>


<div class="left">...</div>
<div class="left">...</div>
<div class="left">...</div>

I didn’t see the bootstrap answer, so for what’s it’s worth:

<div class="col-xs-4">Left Div</div>
<div class="col-xs-4">Middle Div</div>
<div class="col-xs-4">Right Div</div>
<br style="clear: both;" />

let Bootstrap figure out the percentages.
I like to clear both, just in case.

I prefer this method, floats are poorly supported in older versions of IE (really?…)

.column-left{ position:absolute; left: 0px; width: 33.3%; background: red; }
.column-right{position:absolute; left:66.6%; width: 33.3%; background: green; }
.column-center{ position:absolute; left:33.3%; width: 33.3%; background: yellow; }

UPDATED :
Of course, to use this technique and due to the absolute positioning you need to enclose the divs on a container and do a postprocessing to define the height of if, something like this:

jQuery(document).ready(function(){ 
    jQuery('.main').height( Math.max (
        jQuery('.column-left').height(),
        jQuery('.column‌​-right').height(),
        jQuery('.column-center').height())
    ); 
});

Not the most amazing thing in the world, but at least doesn’t break on older IEs.

But does it work in Chrome?

Float each div and set clear;both for the row. No need to set widths if you dont want to. Works in Chrome 41,Firefox 37, IE 11

Click for JS Fiddle

HTML

<div class="stack">
    <div class="row">
        <div class="col">
            One
        </div>
        <div class="col">
            Two
        </div>
    </div>
        <div class="row">
        <div class="col">
            One
        </div>
        <div class="col">
            Two
        </div>
                    <div class="col">
            Three
        </div>
    </div>
</div>

CSS

.stack .row { 
clear:both;

}
.stack .row  .col    {
    float:left;
      border:1px solid;

}

Here’s how I managed to do something similar to this inside a <footer> element:

<div class="content-wrapper">

    <div style="float:left">
        <p>© 2012 - @DateTime.Now.Year @Localization.ClientName</p>
    </div>

    <div style="float:right">
        <p>@Localization.DevelopedBy <a href="http://leniel.net" target="_blank">Leniel Macaferi</a></p>
    </div>

    <div style="text-align:center;">
        <p>☎ (24) 3347-3110 | (24) 8119-1085     ✉ @Html.ActionLink(Localization.Contact, MVC.Home.ActionNames.Contact, MVC.Home.Name)</p>
    </div>

</div>

CSS:

.content-wrapper
{
    margin: 0 auto;
    max-width: 1216px;
}

@Leniel this method is good but you need to add width to all the floating div’s. I would say make them equal width or assign fixed width. Something like

.content-wrapper > div { width:33.3%; }

you may assign class names to each div rather than adding inline style, which is not a good practice.

Be sure to use a clearfix div or clear div to avoid following content remains below these div’s.

You can find details of how to use clearfix div here

display: table;
If text needs to appear
as if on the same line

In other words; if the vertical alignment of text in each <div> needs to be identical, one can attempt a modern retro throwback to yesteryear with the somewhat controversial table styling:

.container {display: table;}
div   {display: table-cell;}

This proved to be quite useful to format CSL-styled citations in Pandoc, as shown below:

div.csl-bib-body {}
div.csl-entry {
    margin-top: 1rem;
    display: table;
    }
div.csl-left-margin {
    display: table-cell;
    }
div.csl-right-inline {
    padding-left: 1ex;
    display: table-cell;
    }

The citation number div and the citation data div are now shown at the exact same height.