http://lygte-info.dk/review/batteries2012/Common18650comparator.php
You need around 3.8v for each LED to be able to hit 5A, which due to driver losses will be around 4v per cell on the input side. At 3.7v per cell, you'll be down to an output voltage of 3.5v per LED, and the max current the LEDs will run at with that voltage is around 3.4A. And it just keeps going down. When the 3 cells are dead flat at 3v each, the max current the LEDs will allow is approximately nothing.
So you see why you can't take a straightforward battery discharge curve, overlay it on the LED Vf chart, and get a solid number out the other end. The battery discharge is done at one set current for the entire run, while in a direct drive(ish) situation, the current is constantly falling as the voltage falls.