<div id="contentContainer">
<div id="leftColumn">
Left Column Content In Here
</div>
<div id="extraContent">
eXtra Content in here
</div>
<div id="rightColumn">
RightColumn Content In Here
</div>
</div>

#contentContainer{
 margin: 0 auto; /* Centers the whole div on the page if you use text-align: center; on body or html */
 width: 800px; /* Width of the main content - change to a % for a fluid design (85-90% is a common favourite) */
}

#leftColumn{
 width: 150px; /* Adjust to suit your design */
}

#extraContent{
  width: 150px; /* Adjust to suit again, but keep same width as leftColumn */
}

#rightColumn{
 float: left; /* Makes the Div ignore all placement above it on the screen */
 position: relative; /* Keeps the Div in place at the top of the parent div (contentContainer) */
 width: 100%; /* 100% width to keep it so you can still see all content within the div */
 margin-left: 160px; /* Change to suit design.  If a fluid design, ensure its a few % MORE than the width of the two divs on the left, else make it 5-10px bigger than them */
}