If you were using the regulator for some low power application it could take 15V or if you had a heat sink with adequate cooling it would also work. But Woodie is right. Don't use a three cell pack as you will ask the regulator to dissipate a lot of power which means heat and if it isn't adequately cooled it will shut down to save itself. It will continue to work after it cools to a point in its normal operating range and it is nearly impossible to permanently burn it out. If you must use a three cell pack add a resistor in series between the battery and the regulator to drop the voltage. The resistor must be rated at more than the expected power dissipation and you will want to cool it too. Easier to use two cells.

Jim