If each of your right and left floats have set widths, then add the appropriate margins to the center div (i.e. if both floats are 200px wide then set the left and right margins on your center div to 200px each)
Make sure all your widths (including margins and padding) add up to less than your screen width (whether you're using percentages or not). Oddly enough, putting the center last allows the floats to go left and right and then the center slides up into the middle. Just remember to clear the floats AFTER the center div.
What I forgot to say was that the center div is variable width. Actualy in the left div there is a logo, in the center div there is a facebook like button, and in the rigth div is copyright info. So the facebook div sometimes is wider, sometims thinner, depends on if you are logged in to facebook or not. So I cannot set specific margin left, thats why I want to center it automatically.
The auto balances the left and right margins of the div. It should still slide between the left and right divs, but you'll need to at least put a max-width on the center div otherwise it will show up beneath the other two.