Drive a 3w led with a 18650 li-on battery
A buck-boost regulator will give you constant brightness and constant current but involves more design.

A Thermal sensor that rises in resistance with temperature will work well with the right part.

I would choose a 350mA to 500mA PTC and attach it to the LED heatsink.
They are rated by Holding current and trip current is 2x but if also heated by LED, it will trip earlier depending on thermal design.

Given the wide variation in Ri (internal resistance) of the 3W LED from 3.2-3.4V @ some rated current like 500mA this is equivalent to Vf=2.8V + If * R1 for Ri = 0.8 to 1.2Ω  adding 0.5 Ω parts in series/parallel as battery drops in voltage then bypass.  Normally a 3W LED is Ri= 1/3 Ω. 

Your specs may vary but it appears to be a 1W LED with 3W pulse capability.  2.6V is dim 2.8V is 10% max current.







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