I concur with the advice USAF52 gave.
Every yr, and I mean every yr., there is at least one poster that placed multiple SA's and ended with the nom to the SA they didn't want.
Fortunately for you SC is not what most would classify as a competitive state. However, you still run that risk of applying to your 3 MOCs and in the end all of them give you USAFA. An academy you just threw in for filling a blank.
If I am correct SC MOC's don't talk. "Talk" = to the issue of spreading the wealth re;noms. In other words they will do everything in their power to give at least one nom to each applicant, that means if you place all 3, and the AFA is the one they have worked out for you, you get a nom from one MOC and it is to the AFA. No nom = no appt. It is crucial you understand that for some this is how it will work. Even an LOA to an SA cannot get apptd with out a nom.
As for the essay, I think it really does not matter about which SA. The fact is they are looking at the big picture, which is not attending the SA, but becoming a leader in the military and serving our country.
falcon is also correct it is really important to play the WHAT IF game in a negative light because it is possible. To say I want to fly X type of airframe is great, but the reality is that you might not get it, thus what will you do at that point?
This will be at least 9 yrs of your life (4 SA, and 5 AD). You will live it longer 365 days a yr as an AD than as a cadet.
Just as you need plan B for the what if re:SA's, you need plan B for the what if re:careers.
Thoroughly investigate each branch.
OBTW DS requested only one SA from each of his MOCs. He got all 3 noms to that one branch. I state this because had he placed 3 different SA's he could have had 1 to each, and he had no desire to attend USNA or USMA or USMMA. For him it was AFA or nothing. By placing only 1, he had 3 slates to compete on at the AFA. The other scenario would have given him 1 slate at the AFA, and 1 at two other SAs.