Example on fuel use.

Say a flight requires 40,000 pounds of fuel, then you fill all 3 tanks equally to 12,000 pounds each, then add the additional 4,000 pounds in the centre tank.
For the flight, I will use Tank #2, to feed fuel to all 3 engines, by opening the 3 fuel crossfeed valves. I will leave it like this until Tank #2 has 12,000 pounds remaining, then I will close the 3 fuel crossfeed valves, and let each tank feed fuel to the corresponding engine - i.e. Tank #1 feeds fuel to Engine #1, etc.

If I do a flight that only requires 18,000 pounds of fuel, then I put 6,000 pounds of fuel in each tank, leave the fuel crossfeed valves closed, and let each fuel tank feed its corresponding engine.

All tanks must have fuel in them (although you can probably still fly it without any fuel in the centre tank (Tank #2)), but it is not how the 727 was designed to use fuel. LOU, a former airline pilot that flew the 727, spoke about it in another topic in this forum (the 727 Captain forum), but I can't find it right now. :-/