Really no way to get nominations across the board other than to put your best foot forward and apply to as many sources as possible. But even that doesn’t make it any more likely that you'll get nominations across the board.
Many states’ MOCs coordinate their slates to spread the wealth, which makes it harder. Also, most MOC interview panels ask you to rank your SA preference. Now, you can try to game the system by giving a different preference to each of three MOC panels, but that doesn’t make it any more likely, because of MOC coordination, the popularity of any given SA in that state/district, and the quantity and quality of candidates for any given SA.
With all due respect, please don’t get greedy. It’s such an ultra-competitive process that you should be thrilled out of your mind to have one nomination, much less several across the board. I have zero problem with a candidate who’s yearning for a nomination to all SAs — or to any SA. My DD was one of those, because she’s genuinely willing to serve in any DOD branch, so great is her desire to become a commissioned officer.
But know that every year, there are many top-notch — nearly perfect, on paper — candidates who get only one nomination, and it’s to their #3 choice. If you really are serious about any SA, to the point of hoping for nominations across the board, then you should be incredibly satisfied with that outcome.