Help with frames!

Hi.

I am creating a page consisting of three vertical Lines (see diagram)

I want to make the main (centre) frame central on the overall page when
viewed on browser, and the two outside frames equal. I tried making the left
and right frames relative and the central one to be 700 pixels but it didnt
work!.

SO
I made the left frame first, with a value of Relative for the left, and 700
for the "right". I then inserted a new frame to make the right frame, which
changed the "right" frame into the central main frame, and set the central
frame to 700 pixels also, and the new right frame relative. But instead of
getting the central frame central to the page as desired, they were more
towards the left!!

Desired look of page when viewed in browser:

Left Main Frame Right
_______________________________

| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
| | |
________|______________|________|
The centre frame HAS to be central on the page and HAS to be 700 pixels

Please help me!!!

Stephen