I tested a XHP50.2 two weeks ago in 6V set-up, that should give you some information too for the 3-battery discussion. Based on those numbers:
Say you want 2A at 12V, that is 4A at 6V for 3000 lumen. You need 6.3V for that in 6V situation, that is 12.6V in 12V situation, so 4.2V per battery under 2A load. So no, 2A is not feasable.
Try 1A, =2A at 6V, so 1800 lumen and 5.95V, that is 11.9V in 12V configuration, almost 4V per battery. With high drain cells fully loaded providing 1A, that should work for a very short period if there are no voltage losses in the flashlight.My conclusion again: does not work.
Or did I make mistakes in the above numbers?