lucy24 - 2:30 am on Jan 1, 2013 (gmt 0)
Do you mean that the two side columns might be different widths, so you can't do it simply by centering the entire table? Let's see what you've tried so far, so nobody wastes their time coming up with brilliant solutions that work in 4 out of 5 browsers, but never the same 4.
I'm thinking something involving absolute positioning and a negative margin of either 50% or half the middle cell's pixel count-- but you just know that some browser somewhere will then eat half the table. And cell width is notoriously user-agent-dependent; they all seem to have different rules about which element is most important.
Now, if you set the table-layout to "fixed" and make invisible headers with the same content on left and right...
What's on the sides of the table? Or does it span the whole page?
... and that's why I said, let's see what doesn't work ;)