Hi Brian,
1 LED with 1 x 420ohm resistor with 9 volt drop across the resistor = 21mA current.
With 3 LED in parallel each drawing 21mA total current = 63mA whilst the voltage across them remains constant @3 volts.
You therefore still need to drop 9volts but at the higher current: -
9volts / 63mA = 142.8 ohms.
A single 150ohm is the closest standard value and would be fine.
For single running the 420 ohm resistor suggested is not an (E12) standard value**, but you can use a standard preferred value of 470 ohm, which will reduce the current slightly to 19mA, but I doubt you would see much difference in brightness.
** 420 ohm is available in the E24 range but not all places stock the E24 range... most only stock the E12 range.
I hope this helps.
Best regards.
Sandy.