According to WDWMagic who are pretty accurate with their information, they are saying that coming later this Spring you will be able to add additional Fastpasses once you complete using your 3 via the App instead of going to a Kiosk in the parks.
Also worth noting that originally when Disney first announced MyMagic+ they mentioned MyMagic+ Storymaker. Storymaker will allow guests to interact with attractions and possibly even characters with use of the Magic Band. Certain possibilities are Mickey knowing your name when you approach him in the Magic Kingdom. Also the most well known example is at the end of It’s a Small World where they might say your name and goodbye in your native tongue.
This is a great news for enhancements to a great experience in the parks. Let’s just hope that they also plan to allow Fastpass+ selections to happen in multiple parks on the same day in the future.