Your source voltage is 6V. If you connect them in series, it may be a bit "dark" because the forward voltage of the white LED is around 3V.
If you connect them in parallel, what you need to do is to limit the current to around 20mA to 30mA. The resistance of the resistors (of each LED) should be around (6-3)V/(20mA or 30mA) = 150 to 100 ohm.