? About Multi Floating Div's

I want to float three ' Div's ', >> Left Col, Main Content, Right Col <<, within a ' Wrapper Div ' .

Example :

<div id="wrapper">
  <div id="left_col"></div>
  <div id="main_content"></div>
  <div id="right_col"></div>
</div>

I understand floats, but the question is, would I float all left? Or float Left Col and Main Content to the left, and the Right Col right?

Also+++ Do I have my Div's within the Div Wrapper correctly in order? I ask because I see that some people always put everything floating to the first and then move towards left.

Thank you....