Each MOC can nominate you to any or all of the SAs. However, they must nominate you individually for each specific academy. IOW, you would receive a nom to USMA, a nom to USAFA, a nom to USNA, etc. They are not interchangeable.
In PRACTICE, most MOCs will only give you a nom to one SA. Most require you to pick your first choice or rank your choices. I've never heard of an MOC who nominates a candidate to a SA in a situation where the candidate said he/she didn't want to go there. However, if you indicate you would accept a nom to more than one SA, you may get your second (or third) choice.
In geographic areas (states or districts) where there is a lack of qualified candidates, an MOC may nominate the same person to more than one MOC if the candidate has expressed an interest in more than one.
If you are only interested in one SA, that is the only one you should put down and that is the only one for which they will consider you. If they ask you if you're interested in other SAs, and you're not, there is no harm in saying so. Be sure to do so without denigrating the other SAs.
Also, most (if not all) MOCs have only one SA nomination application packet because the information they need from you is the same regardless of which SA you want to attend. Thus, if you fail to complete your packet, you will ruin your chances of receiving a nom to any SA.
