Using fuel tank names for L-1011 for FSX/SE:
Left Tank feeds Left Engine (Engine #1)
Right Tank feeds Right Engine (Engine #3)
External Tank 1, and External Tank 2 feed Centre Engine (Engine #2)

Using fuel tank names for L-1011 for P3D4/5:
Tank #1 feeds Left Engine (Engine #1)
Tank #3 feeds Right Engine (Engine #3)
Tank 2L, and Tank 2R feed Centre Engine (Engine #2)

Because of this, I have equal amounts of fuel in all of the fuel tanks.
By that, I mean that Left Tank (Tank #1), and Right Tank (Tank #3) would have, lets say, 20000lbs, and External Tank 1, and External Tank 2 (Tank 2L & Tank 2R) would have 10000lbs (a combined total of 20000lbs).
The P3D4/5 tank names are placed in brackets.

Under the  above circumstances, I don't need to have the Crossfeed Valves Switches for a flight.

Note. If I'm going to do a flight in the Captain Sim L-1011, I prefer to fly the 1011 Captain for FSX/SE in FSX/SE (32bit) rather than the 1011 Captain II for P3D4/5. :D ;D