Three LEDs with Vf of 3V is 9V in total.
Remember take your batteries low and high Voltage. For a fresh li-ion that would be 4.2V and I don't know where you consider discharge is, but, let's say 2.5V.
For three batteries that would be 7.5V to 12.6V.
In order for the Shark buck to be in regulation all the time the complete battery range must be higher with margin in order to maintain regulation.
Since 9V is not always less than 7.5V - 12.6V this situation is not optimum for full regulation all the time.
It may be in regulation under fresh batteries conditions, but, will fall out of regulation at some point in time before the batteries are fully depleted.
To determine a different configuration determine the following:
1) sum up the Vf of all the series LEDs.
2) Based on the battery technology and number of batteries determine the low and high battery voltage.
3) For a boost the low and high must be less than the sum of Vf. For a buck the low and high must always be higher than the sum of Vfs.
Wayne