Since you'd prefer to use only one of your mobo's CHA_FAN headers to control six 4-pin fans, I suggest you get a HUB instead of a Splitter. Here are two examples at $15, versus those Splitters at $7 each.
https://www.amazon.com/DEEPCOOL-FH-...b&qid=1566153889&s=home-garden&sr=1-2-catcorr
https://www.amazon.com/SilverStone-...b&qid=1566153889&s=home-garden&sr=1-1-catcorr
Splitters and Hubs are different devices. With a SPLITTER all its attached fans (you suggested ones with 3 outputs each) are connected in parallel to the mobo header outputs and they draw all the fan motor power from that header. A header can provide up to 1.0 A max current to its load, and three such fans fits within that easily, but not six. So you are right - you'd need two Splitters to use three fans per header. A HUB does it differently and really only works with 4-pin fan systems, which IS what you have planned. It gets its fan speed control signal from one mobo header and distributes that to all its connected fans. But it gets the POWER for those fans directly from the PSU via a cable that must plug into a PSU power output connector - in both of those examples, a SATA power output. This can supply much more power than a mobo fan header can, so a Hub does not overload the header and can handle more than 3 or 4 fans. To work, the Hub MUST receive the PWM signal from the mobo header, so make sure the header you use is set to use the PWM Mode for control, and not the older DC Mode.
Any mobo header can handle the speed signal fed back to it from only ONE fan. So any Splitter OR Hub will send back the speed signal from only ONE of its fans and ignore all the others. The Hub will have ONE of its outputs identified as the ONLY one whose speed is sent to the mobo, so be sure that one of your fans is plugged into that port. This has NO effect on ability to control fan speeds - all the fans are identical in your case and all will receive from the mobo / Hub exactly the same control signals, so they will do the same thing. But you should understand one small impact of this. A mobo fan header has a secondary function - monitoring the fan speed signal for FAILURE - that is, a too-slow or missing fan speed signal. When you use a Splitter or Hub, most of the fans' speed signals go nowhere, so those ones can NOT be monitored for failure. It just means that, from time to time, YOU should check that all your fans are still working.
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