As far as I know....

Tank to Engine at engine start (all the switches are already set that way when the 727 is first loaded into FSX).
Possibly Tank to Engine during the climb, but if not, Center Tank to ALL Engines.
Once at cruise altitude, feed all engine from the centre tank until it is equal to the fuel remaining in the wing tanks, then switch it back to Tank to Engine for the rest of the flight.

Note. If all fuel tanks are equal when you start the engines, leave them that way for the entire flight.

To set it up so that the center tank to feeds all the engines, set the switches like I have them set in the following image:

http://i669.photobucket.com/albums/vv58/Markoz958/727fuel.jpg~original