Do what is more fun. Scribe job is great and all, but there's only so much you can take away from that. You seem to already have a couple of years of experience, so enjoy your time while you can.
Defnitely do the national team internship because it shows commitment. Medical schools LOVE to see commitment, and you already have enough scribe experience. 🙂
The U.S. National Team internship will be a far more exciting addition to your activities section than a third year of serving as a medical scribe! It could set your application apart. You can connect the activity to medicine if it involves team work, communication, dedication, etc. The way you write about it will make all the difference. Special talents and hobbies help people get through medical school; it allows you to maintain more balance in your life. Do what you love!
For ideas on how to write about last minute activities as well as hobbies, please see this article on the topic.