The three c-cell requirement came about because the Scouts originally were tubes of delrin threaded to use UK SL4 heads. The SL4 was designed to take 4 c-cells and it was believed that using same head on scouts but underpowering it with only 3 cells would make it less likely to burn out when switching it on. Using 3 c-cells has no value on its own as a rule, it is dependant on the bulb.

LEDs need to be powered by the right voltage or they do not work, they are not like incandescent bulbs. They also have no risk of burning out when you switch them on so underpowering them is not an issue.