It could be a matter of your DIVs not being in the right order within your HTML. Try this:
content
left sidebar
right sidebar
I also notice that content and rightbox have a different parent than leftbar, which might cause some problems. You should have all three DIVs in the same parent DIV, like this:
<div id="main">
<div id="content">...</div>
<div id="leftbar">...</div>
<div id="rightbox">...</div>
</div>
Layout Gala, which I didn’t know about until just now, already has some of these layouts. Give one of those a shot and let us know if you still have trouble.