When you do the online check in, you select a funding account for each person separately. The process will go through each person, and you do not need to do anything in advance.
This has nothing to do with creating a carnival.com online account. Completely separate. No need to create Carnival.com accounts for anyone. Won't do anything, won't save time.
I am managing multiple rooms for the first time, also. But, from what I've read here, you can pick the arrival time of the other bookings you are managing, so you can pick the arrival time of all 3. However, you do have to do them sequentially, so, I guess it is possible that the time you select for the first one you do may not be available for the second or third.