I just tried this using my AP to make FP+ reservations for a random day in March. After making my 3 picks, I went back to cancel, and was able to cancel one of the 3, leaving me with 2 still valid.
I don't think I could do this before. I was always able to change any one to a different time or a different experience, but I think if you wanted to cancel, it was all or nothing.
Edit: My next step was to go back to see if I could make my third FP+ selection for that day at a different park. But, that didn't work. When I tried to add a reservation for Space Mountain, it told me I had only made one selection, and when I continued they tried to throw me two others at MK instead of adding SM to the 2 I have at DHS.
Maybe that will be the next step.
Anyway, now I have to go back and cancel those reservations I won't be using.