How do I code a third key
 

Hi guys. A while ago I lost one out of my oiginal two keys and got another replacement (cut according to the keycode)but it wasn't incoded. It works in the doors and fits in the ignition switch but won't start the car. Then about3 months laterI found the original key so now I have both originals and a third key which needs to be incoded. Is there any possible way I can incode the third key if Ihave the two originals without having to take it to a dealer? By the way the car is a 03 323 protege.