I'm afraid I don't understand why this would be. If you book three FP+ for all but one party member and only 2 FP+ for that party member, my understanding is that it's all the same once all prebooked FP+ have been used. Granted, I've not played around with FP+ much in a group setting so I acknowledge I may be missing something, but when I traveled in 2018 I booked only one FP+ for myself and I was able to immediately start booking more of them in-park as soon as I'd used it.
If anything, it seems to me that the party member with fewer FP+ booked to their account would probably be able to book additional FP+ in the park sooner than the rest of the party (assuming the ride they don't do is everyone else's third).