The vast majority of single cell “budget” lights aren’t designed for anything. They’re direct drive, so whatever voltage the cell is, that is available to the emitter (minus any resistance losses) and you can look in the data sheet for the emitter and see what current it will draw at a given voltage. (which you know)
Now... if that is the case with this light, you have to multiply that by 3 to see the massive current that this light could draw (26650s don't sag much - think 3.6v @ 10A on fully charged cells at turn-on) since the LEDs are wired in parallel. (Resistance will certainly play a larger part than it would in a multi-cell config)
If this light is not direct drive, its going to be using a driver like nothing else out there. I'm just going to go on record (so I can be wrong later) and say that I think it's DD and the current could be north of 5A (closer to 10A). Although I admit, I don't know how the light would even be functional on a typical protected 18650. Otherwise,its going to be a 8x7135 and be way underdriven.
As far as efficiency, look here at the XML brightness @ 1A and multiply it by 3 and then look at the brightness @ 3A