Do you have more than one fan reporting RPM back to the Aquaero?

What method are you using to control the fans, power or RPM?

Okay, using a 3-way splitter on the fans means you are sending 3 different frequency signals to the one fan header and confusing the aquaero. While you can power more than one fan from a single fan header via a splitter, you cannot report more than one fan frequency. You are trying to report three at once, hence why you are seeing fluctuating readings. This has nothing to do with PWM which is something entirely different.

Can you send a picture of your splitter.
if its the splitter for aquabus then its what cc01 said. (this splitter splits all cables, for fans you need a splitter that splits just 2 cables and the remaining cable is only going to 1 fan)

i actually don't know about the internals of that splitter.
but since its made for fans i have to guess that it only lets 1 fan return its speed signal.
try to switch places of which fan is on which header of the splitter. (maybe 1 fan is faulty)
if that doesnt work, just connect 2 fans and not 3 (you seem to be using a lot of power).

I think I had a similar splitter as that one once.....it has traces that connect all 3 signals from the fans to the output header. What you can do is scratch the rpm trace all the way through between the 1sr & 2nd fan so that only the first fan closest to the output header is producing an rpm signal to the output header
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Like was said, only one fan in a group can report rpm or else the device has no idea what is going on.

A good thing to do is educate yourself on fan wiring and learn how it works. People that have a lot of fans in their rig usually learn how to make their own wiring harnesses. For example I have several fan harnesses I made that are connected to 8 fans. In each case the rpm wire is picked up from one fan only and sent to the fan controller.

I have seen lots of fan splitter devices that are made wrong, you have to be careful what you buy.

When I use splitters that connect more than one RPM cable all I do is depin the RPM wire from the fan connectors on all but the first fan. Its quick and easy and the pin can just pop back in if you want to change it later.

That's it.