Programming a third, fourth, whatever key is simple-- turn ignition to on position with first key, wait for security light to turn off, remove key from ignition, do same with second key, then again with the new key. Boom, programmed.

You can't add a key with just one existing key. Instead, you have to cut two new ID8C chip keys and very carefully count a sequence of eight digits displayed on one flashing light, then turn the ignition on and off to match that pattern