Look for Supermicro fan control scripts here. From their documentation, you'll see that Supermicro boards have two zones for fans, which can be set independently of each other: That's your numbers vs. letters…
Then, depending on your case and how the build is arranged, you might actually want to put the CPU fan on 'A' and use '1', '2', '3' for case/drive cooling, adjusting the script as appropriate.