Would it also possible to change the fans of the miner to standard 120mm computer fans to make it even quieter, or does Braiins OS doesn't work then anymore? I ask this question because I think I will use the miner to run all 3 boards at 100 Watts each if this is possible with basically no noise.
No, you can't replace it with an incompatible fan you should only replace it with the compatible fan but if you want to test it in Braiins OS maybe the fan will always run at full speed by enabling immersion mode.
Why don't you just adjust the fan speed at a very low speed(The stock fan) just make sure to monitor the temp but since you set the power limit to 100w I think it won't get hot too much.