When I've done this in the past I think I did something like this:
1) Declare a wrapper div around the content of the centre panel. Set this to 100% width.
2) Inside that div, add another div with the centre panel content. Set the left and right margins to be that of the width of the left and right panels.
3) Add the left div, float it left and set the width to the relevant value.
4) Add the right div, float it right and set the width to the relevant value.
1 & 2 fill the screen, but leave space for the left and right columns.
3 & 4 define the left and right columns,
If that's not good enough, I may get a chance to look into more detail later.