What is my error in this CSS float to column code

Question

I just want to make the three articles in 3 columns, when I code it using dreamweaver on design mode panel, they are in three columns but when I preview it using live view or browser its doesnt appear in three columns, what possible code mistakes did i do?

CSS

.3col {
        left:0;
        float:left;
        top:0;
        height:100%;
        width:200px;
}

HTML

<section id="otherArticles">
    <article>
      <div class="3col">
      <header><h3>Lorem ipsum dolor sit amet</h3></header>
      <p>Rutrum vitae vestibulum condimentum metus. Donec magna leo sapien augue tellus, phasellus erat, adipiscing lectus phasellus enim nulla, eu aliquam sodales</p></div>
    </article>  

        <article>
      <div class="3col">
      <header><h3>Lorem ipsum dolor sit amet</h3></header>
      <p>Rutrum vitae vestibulum condimentum metus. Donec magna leo sapien augue tellus, phasellus erat, adipiscing lectus phasellus enim nulla, eu aliquam sodales</p></div>
    </article> 

        <article>
      <div class="3col">
      <header><h3>Lorem ipsum dolor sit amet</h3></header>
      <p>Rutrum vitae vestibulum condimentum metus. Donec magna leo sapien augue tellus, phasellus erat, adipiscing lectus phasellus enim nulla, eu aliquam sodales</p></div>
    </article> 
</section>

Answer 1

HTML IDs and CSS Classes can't start with a digit! Change 3col to something like column.

Here's a simplified example:

CSS:

<style>
    .column {
        float: left;
        width: 200px;
    }
</style>

HTML:

<div class="column">
    <h3>Test</h3>
    <p>Rutrum vitae vestibulum condimentum metus...</p>
</div>