Invisalign wearing for 3 weeks not 2 weeks
Invisalign recommends changing trays every 2 weeks and that is the protocol I follow when discussing the treatment with a patient. I am not sure why your dentist would want you to change every 3 weeks. Call your dentists and ask him for the reasoning
Some teeth movements are more difficult than others, so sometimes they require more time to move. 2 weeks is the national average. My advice is to listen to your orthodontist, he may have a good reason for the longer interval time.
Invisalign aligners are worn for 2 weeks
The official protocol for aligner wear is 2 weeks each. That is the standard which all data and research used to develop the method is based on. I am not aware of any advocacy of 3 weeks per aligner and am not aware of any benefit. It sounds like a great question for your dentist that is advising that. It is possible that this is their own personal protocol and may be based on their own experience. It doesn't make it wrong.
Invisalign does suggest a 2 week protocol. Some people condense it down to every 10 days although that is not recommended. You will have to consult with your dentist why he likes to extend it to 3 weeks. Maybe he is giving the teeth a chance to settle in more before jumping to the next tray