If all of the leds are the same you may as well connect them in series then you only need one resistor.
This is the most simple method IF you have a 9VDC supply.
Do you have data on the forward current and voltage across the leds?
So for example lets say the voltage drop is
2V per led, so the voltage across all 3 will be 6V in series
You have a 9V supply, typical current for the leds is say for example
20mA
So 6V across leds with 9V
(is it regulated? to be sure measure the volts with no load) leaves 3V across the series resistor at 20mA typical (specified current depends on the LED)
Ohms law R=V/I = 3/20x 10⁻3
3000/20 or 150 ohms
To be on the safe side start with a higher value R say 300Ω and measure the series current, (multimeter required on suitable range) adjust value of resistor for the correct current (or tweak until leds reach max brightness and are about to blow up then back off a bit - that part was a joke- don't do that)
Resistor wattage should be OK with a 1/4 watt resistor or even less, (you might get away with a 1/8 watt depends on LEDs but check if it gets warm - (resistor wattage can be calculated by current through resistor x volts across it or V across resistor squared /R )
some info
here
