You can cancel individual FP+ choices on the MDE website (I presume the MDE app too, but I don't have my phone handy at the moment).
After you've made them, go into the FastPass+ section and click on the "Cancel FastPass+" link. Choose the date and it'll give you the option to individually select FP+ choices to cancel.
Just be aware that I haven't been able to figure out a way to re-add just a single FP+ choice in that now-empty slot without cancelling the entire day's FP+ and starting over. So unless you *really* know that you won't need to use any other FP+ choices, it's easier to leave them so that you can change your choices later.
One thing I wonder about... If you cancel one of your FP+ choices for a park that has tiers (such as Epcot), use the other two and then go to a kiosk, will it allow you to make a tier-free choice like the rolling 4th FP+? Or are you forced into the limitations of the tier that your now-empty original-three FP+ slot was for? (Essentially making the first kiosk-based FP+ your third FP+)
-Rob