Hi Huutchh!,
I've checked the user manual and found out why you are not succeeding with your procedure:
in order to program a third new key you need to have BOTH the working keys with you!
Let's read the manual:
Programming with Recognized Transmitters
A new transmitter can be programmed to the vehicle when there are two recognized transmitters. To program, the vehicle must be off and all of the transmitters, both currently recognized and new, must be with you.
Now you know why it's not working! You'll have to visit a GM dealer and ask for a reprogramming. I guess a Vauxhall dealer could do it, too..
See you!!,
Eos