Alignments (something like colspan)

I’m trying to make align stuff like… two columns side by side aligned left (works) with the div beneath them to center beneth the double column
Using a table:

[CODE]
<TR>
<TD></TD><TD></TD>
</TR>
<TR COLSPAN=2 ALIGN=”center”>
<TD></TD>
</TR>[/code]

So… how do I do this in CSS?
Do I need to impose widths on all the divs?
If yes, how do I measure how many ems in a div?
To get the third div to drop below the others, is “clear:left” the proper method?
Thanks!

The code I have so far:

[CODE] DIV.left {
position: relative; float:left; top: 0; padding-left:1em;
}
DIV.right {
position:relative; float:left; top:0; left:0; padding-left:3em;
}

DIV.bottom {
position:relative; top:0; clear:left; margin-left:5em; margin-right:auto;
width:100%; text-align:left;
}[/CODE]

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