How much resistance should I use?
IR 940nm LEDs normally have a Vf of about 1.2V at 20-30mA. Given that the specs are a bit on the 'light' side, let's design for 25mA not 30mA. If it has to survive high ambient temperature or needs to be especially reliable you probably should use a lower current (and buy the LEDs from a better source with a real datasheet).
So with the 3 LEDs in series you'll have ~3.6V. To get 25mA from your 9V battery means that there will be 5.4V across the resistor. So your resistor can be 220 ohms. Power dissipation in the resistor is 135mW so a 1/4-W resistor is fine.