You don't want to cancel one FP+, what you need to do is modify it. Just pick the fastpass you want to change and then click, modify time or modify experience. It will let you pick just one person to change. I did this so that half of our group could do one ride and half another.
If you cancel the FP it will not let you add back just one. You will have to cancel all three and start over, or you will be stuck with just two.