Some more detailed info to help you plan.
There are two major fan designs in the market. The older design uses a 3-pin connector (the fan's cable has a female connector with holes), so it's called a 3-pin fan. The method of control of this design is to vary the VOLTAGE supplied to the fan, from 12 VDC for full speed down to about 5 VDC minimum - any lower voltage may cause the fan to stall and not re-start. On these fans, Pin #1 is Ground (Black wire), Pin #2 is the +VDC power supply line (Red wire), and Pin #3 is the speed signal sent by the fan back to the mobo (Yellow wire).
The newer design are called 4-pin fans or PWM fans, because they use a 4th pin to carry the new PWM control signal. The other three pins have almost the same signals as the 4-pin one does, except that Pin #2 does NOT vary its voltage - it is always 12 VDC. Inside the fan is a small chip that uses the PWM signal to alter the flow of current from that 12 VDC Pin #2 through the motor windings to change speeds. The wire colours on a 4-pin fan are NOT the same as a 3-pin one.
The PWM system design tried for some backwards compatibility with the older 3-pin ones, but there are odd limits if you mis-match fan type and mobo header type. If you plug a 4-pin fan into a 3-pin header, it still will operate under speed control, even though that is not the ideal way to control such a fan. If you plug a 3-pin fan into a header using the 4-pin PWM Mode for control, that fan will always run full speed - you cannot change the speed that way.
Virtually all fan headers can supply up to 1.0 A max current in total to all of the loads connected to that header. Most common fans now will consume at max 0.10 to 0.25 A, so three or even 4 fans on a single header is acceptable. In planning, if at all possible find the spec'd max current for each fan you use. IF you find only a Watt spec (not amps), Watts = volts x amps, and max volts is always 12 VDC.
Splitters and Hubs are different devices, but both are used to connect two or more fans to a single header, and all fans in the group receive exactly the same signals. IF they are identical fans, they all will do the same thing. But if they are different, they will operate at different SPEEDS, but that does not matter. The control system does NOT try to achieve a particular speed. In fact, it does not care what the fan speed is - it concentrates on the TEMPERATURE of the item the fans are trying to cool.
A SPLITTER is a simple device that connects the Ground and +VDC lines from the mobo header to all its fans in parallel, so all of the power for the fans can come only from that header. Thus you are limited to the 1.0 A max load. But because it operates this way, the varying Voltage from Pin #2 (IF the header is using the older 3-pin fan Voltage Control Mode) IS sent to all the fans, and this device CAN be used to control the speeds of 3-pin fans. By the way, you CAN use a 4-pin Splitter with 3-pin fans, so today you will find it hard to buy a 3-pin Splitter.
A HUB is different. It has an extra "arm" that must plug into a power output (SATA or 4-pin Molex) from the PSU. It uses the Ground and +12 VDC power lines from that source to distribute the constant 12 VDC power supply via Pins 1 and 2 to all it fans, and does NOT draw any power from the mobo header. Thus it avoids the 1.0 A limit, and can supply much more power - typically, up to 4.5 A. It does get the PWM signal from the header's Pin #4 and distribute that to all its fans, but that is NOT a load-limited problem. HOWEVER, this means that all the fans connected to the Hub are getting ONLY the signal system used for the new 4-pin PWM fan design type. It can NOT control the speed of a 3-pin fan.
A small note that applies to both Splitters and Hubs. The speed signal generated by a fan and sent back on Pin #3 is a series of pulses 5 VDC high, and two pulses per revolution. The mobo header counts these to display speed. It also monitors that signal for NO pulses, and interprets that as a fan FAILURE, which triggers warnings and maybe further action to prevent damage from overheating. But that counting circuit can only deal with a pulse train arriving from ONE fan - more than one fan's signal overlapping causes major errors and trouble. So all Splitters and Hubs will send back the speed signal from only ONE fan, and ignore the others. You will never "see" the speed of those "others". On simple Splitters, the common method is simply to omit Pin #3 in the male outputs for all but one output connector. On units that look like printed circuit boards or closed boxes with holes and ports, one of those will be marked as the only output that sends back the speed signal. NOTE that, since the speed signal of only ONE fan gets back to the mobo, it cannot monitor ALL the fans for failure. From time to time YOU should check that they all are still working.
Almost all mobos now use only 4-pin male headers, but that does NOT tell you what method the header uses to control fan speed. Instead, to make things adaptable for both fan designs, almost all include in BIOS Setup where you can configure fan header parameters an option to select either Voltage Control Mode (aka DC Mode) required by 3-pin fans, or PWM Mode ideal for 4-pin fans. YOU get to choose. But that also means that you generally can NOT mix 3-pin and 4-pin fans on the SAME header, because each type requires a different MODE setting.
Now back to the specifics of your system, OP. I am assuming for now we are dealing only with your case ventilation fans, and not any part of the CPU cooling system. You have one CHA_FAN header, three PWM fans, and plan up to three more of that type. I urge you to consider only PWM type (4-pin) fans. For only 3 fans, either a Splitter or a Hub can do the job, and a Splitter is cheaper. BUT for sure you cannot connect six fans to one header, so you WILL need a Hub when you get there, and you CAN use that Hub now. The Silverstone item linked by EyyMunchian above is a good example, and has all the output ports you need.
The item linked above by dorsai is quite different. It does NO automatic speed control the way a mobo header does, and it does NOT feed any fan speed back to the mobo, nor does it have any way to tell you what the fan speeds are. YOU are the controller who must decide what speed to run your fans as you change your workloads, and then reach around to the knobs and adjust them. It actually does not use the new PWM Mode to control the fans. To make it work for BOTH fan types, it uses only Voltage Control Mode, and relies on the backwards compatibility feature of a 4-pin fan to operate that way if necessary.
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