WIth two keys you can program a new 3rd key without the dealer. Please see below:
How-To: Programming PATS Keys
The only problem is that you will have a different key cylinder to use between keys 2 and 3.
The way I would do this is to get a plain Jane key cut using the new lock cylinder.
Put the new cylinder in the car and then hold each of the old keys against the ignition cylinder as you turn the plain Jane key. This would simulate each of the old keys in the ignition cylinder. Then just pull out the plain Jane key and use the new chip key.
You may want to get a fourth new chipped key made so that you have 2 chipped new keys made for programming a new key later on.