The voltage of the LEDs is 3.6Vdc but the resistance depends on the power that these dissipate, there are them in different power. You must measure the current flowing through one of them or one of the resistance that is the same if they are in series and having the voltage that falls between their terminals you calculate it. Resistance in ohms is equal to the voltage that falls on it, divided by the current flowing through it. To give you in ohms you must use Volts and Amperes as measurements since a resistance of one ohms is one that when an Amper circulates, a Volts falls between its terminals.