When you have several fans connected to the one header the tacho signal will jump up and down, this is normal. With the 3x3pin power board, you are effectively confusing the tacho input with three different signals at once, hence the unstable signal. To get a stable tacho signal for each fan you would have to connect only one fan per header on the aquaero, which means a maximum of four fans on the aquaero.
No idea why your Corsairs are humming ... Try connecting just one to each output instead of three and see if you still notice the humming. You can adjust how much power the fans receive from the aquaero in the aquasuite software. You have multiple control options to choose from provided you have temperature sensors installed and it will adjust the fan speeds up or down according to the input data.
Its normal for individual computer case fans to show different rpm data at the same voltage as they are not manufactured to such strict tolerances. A fan rated at 1200rpm at 12V, for example, might show 1250rpm and another fan might show 1190rpm. To keep the manufacturing costs of the fan down they are not made to meet precise tolerances and in terms of performance there would be little difference between the two. The fan specs should probably say 1200rpm +/- 10%, and it is nothing to worry about. It doen't mean your fans are faulty. It is also normal for some fan headers to slightly drop voltages, so at 100% output a 12V header might only show 11.9V. Again, I think it has to do with manufacturing tolerances, or it might be because there is a slight voltage drop at a physical connection such as where you plug a cable into a header. I'm not entirely sure but if anyone else could share some understanding of electronics, I would like to know the exact reason for this too.
Hope this helps.