Here is another very simple idea for you to try:
Use 1 Battery to drive directly(no resistor!) 3 X IR LEDs in series.
This way you get no losses at all.
The maximum voltage you can get from a fully charged Li-On battery is 4.2Volts
Each LED will get a maximum of 4.2/3= 1.4Volts.
So it is safe for the LEDs.
With the battery voltage dropping with use,
the LEDs will get less and less voltage and illuminate less.
At 3.7volts each LED will get 3.7/3=1.23Volts
At 3.3volts each LED will get 3.0/3=1.1Volts
Worth a try,
If you do try it, what do you get?
Use 1 Battery to drive directly(no resistor!) 3 X IR LEDs in series.
This way you get no losses at all.
The maximum voltage you can get from a fully charged Li-On battery is 4.2Volts
Each LED will get a maximum of 4.2/3= 1.4Volts.
So it is safe for the LEDs.
With the battery voltage dropping with use,
the LEDs will get less and less voltage and illuminate less.
At 3.7volts each LED will get 3.7/3=1.23Volts
At 3.3volts each LED will get 3.0/3=1.1Volts
Worth a try,
If you do try it, what do you get?