Selection is a numerical equation. 60% PAR, 20% EC, 20% Recs...there is no% given for the fact that all 3 MOCs placed you on their slate.
Coffeecup is correct, multiple noms give you multiple chances of getting in compared to 1.
Here is an example for a candidate for an SA that is on congressional and one Senators slate and applied to multiple SAs:
Candidate 1: 6 noms (Pres, VP, 3 MOCs, and ROTC). Charged to Congressman District 1
Candidate 2: 5 noms (VP, 3 MOCs and ROTC). Charged to Congressman District 2
Candidate 3: 5 noms (Pres, VP and 3 MOCs). Charged to President
Candidate 4: 4 noms (Pres., VP, Sen., 1 congressman) Charged to Congressman District 3.
Candidate 5: 3 noms (VP, Sen A, 1 congressman) Charged to Congressman 4
Candidate 6: 3 noms (VP Sen A, Congressman 1, 2,3, or 4): Charged to Senator A.
Candidate 6: 3 noms (VP, Sen A, ROTC). Charged to ROTC
Candidate 7: 3 noms (Pres., VP, and Sen A). Charged to VP
Candidate 8: 2 noms (VP and 1 MOC) makes it in from the NWL.
Candidates 9 and 10 hope that candidates 1-7 elect to turn down the apptmt. to your SA so you can get in.
As you can see if you only had Sen A as your nom, you could only get in one way and would have to hope that others with higher WCS had multiple noms. If you have multiple noms than you could get in from multiple ways.
The reason the mass mailing occurs is because they are making a giant jig saw puzzle and have no idea what the picture will look like. Many candidates apply to multiple SA's. Look around the board and you will see posters with 2 or more LOA's. They will have to eventually remove their name from one of the SAs. Luigi's son is an example, he was accepted to AFA and CGA, he opted CGA, which meant a spot opened at the AFA. If you are on all 3 of their MOC's slates, your chances will increase.
I have been on this board long enough and can tell you that there are those who get NO MOCS, only a Pres or VP and get in, and there are those with 5 that do not get in. Your chances will increase, but in the end of the day it is your WCS that will get you in. Also realize that @3K candidates have noms, but only 1600 get apptd.