Wow..a lot of hand wrangling and speculation about what Admissions wants. Yes, the statement that Admissions wants Candidates to apply for every nomination they can is accurate, and yes, having more nominations give Admissions flexibility about where is charged. However, I tend to agree with '85 --once Admissions has pulled the trigger and offered an Appointment , they probably aren't going to think twice about that Candidate and move on to the other 5000 they have to deal with.
One additional thought - what does OP tell the MOC panel when they ask "Why are you here if you already have an appointment to all three SA's? Why are you wasting our time ?". If I were sitting on the panel, I sure wouldn't be impressed by the answer "Well, I read on SAF that I should go through with this nomination to give the Admissions office more flexibility on where to charge the nomination." First, in my dealing with local MOC's offices, they really don't care about the internal mechanics at USNA Admissions, and from a practical perspective giving USNA the flexibility to move the DV nomination to the MOC slate could potentially take away an opportunity for another person in the District /State to attend a SA as well. (From the MOC perspective, the more they can send from their District/State, the better !)
Finally, if you are going to ask anyone ..it is probably the MOC office that you would call....ask them if its okay to take their time to accomodate and give the SA some more flexibility.