Inside APW9, have 3 connector for this 3 fan... So, the psu don't read the fan speed ! each fan have only two wire, so no "wire" for tachymeter !
I thinks, the psu have security for "fan lost", and i thinks only read intenssity at each fan, if on fan "show" no amper, so no power, the psu enter in security mode !
IS NOT POSSIBLE left open the psu, the reason is simply, the way of air flow is totaly wronk ... but, i have inspected this, and, the top right and and the midle right is the most high temp zone ... but all psu need fresh air ...
i thinks is possible make two fan 120mm on the left, and on litle more power on the top right
All is purpose... at your hown risk, i don't tested this