Two issues here:
1) General Anaesthetic. Never heard of not being able to drive for 3 days because of a GA. However if this is what you have been told you must do as they say.
2)Knee Op. As mentioned earlier by other members, you may not be able to control the footpedals properly. It takes time to get total movement back, especially when trying to do an emergency brake.
You need to be happy that you can control a car before you drive again. This may take considerably more than 3 days. If you are in any doubt DO NOT DRIVE.
If you need any advice on this be sure to speak with your GP. Better safe than sorry.