Rosario -

As I understand it from guys who have actually flown the 727, you fill the tanks evenly until the wings are full, then keep filling the center tank as needed. Filling only the center tank would put undue stress on the wings...big heavy fuselage and light wings is not good.

For takeoff and landing you're supposed to feed "tank to engine" for safety (so that failure of one fuel system doesn't rob all of your power).

So, as Mark indicated you should set the fuel panel as follows:

1) Feed "tank to engine" (crossfeeds closed and all boost pumps on) for takeoff,

2) After takeoff, shift to burning center tank (crossfeeds open and wing tank boost pumps off) until tanks are all equal.

3) Then go back to feeding "tank to engine" by closing the crossfeeds and turning the wing tank boost pumps back on until you land.

Of course, it's just a simulator, so if you want to run center tank only you can set it up as discribed in #2 above from the beginning. You could also just fill the wing tanks and run the center off of them by opening the crossfeeds and turning off the center boost pumps. Just not how it's done in the real world.