Don't you just need one reference or one loudness limit?
Assuming you have some kind of sound isolation so one sensor can only 'hear" one fan... There is a circuit called a peak detector that "captures" and holds the peak for a period of time (depending on the R-C time constant). One of those on each analog input will hold the peaks long-enough to read several every time through a loop.
Some sound sensors have this built-in (usually called an "envelope follower" but the built-in one may decay too fast.
Or since I assume the noise is fairly constant, you can probably cycle-through the readings one at a time. But you still need to find the peak or average loudness.
Your sound sensor MIGHT have a digital output so you can set the pot to trigger the output when a certain loudness is exceeded. In that case it can probably drive an LED and you might not even need an Arduino!
...I make sound activated lighting effects and I use a peak detector because it's "easier" than reading a waveform and finding the peaks or the average and it means that I can read the "loudness" about 10 times per second (perfect for my application) instead of reading the waveform thousands of times per second. and it frees-up processor time to run the lights.