Mostly right above, but a couple changes.
There are three types of devices for controlling fan speeds. A SPLITTER merely connects all its fans in parallel to the power source and signals from one fan header, so the header does not even know more fans are connected. A HUB does a similar job BUT with an important difference. It gets power for all its fans directly from the PSU (and that means it requires a connection to a PSU output), so the 1 A max current limit of the header does not matter. The Hub gets the PWM control signal from a mobo header and shares that out to all its fans. BUT that means those fans MUST be of the newer 4-pin PWM style that can use that signal. (There is ONE Hub that is different in this matter.) A CONTROLLER similarly gets power from the PSU, but has all its own "smarts" needed to do the control function and does not use a mobo header's signal. Many Controllers are used in conjunction with a software tool to tell it what to do, and that communicates with the Controller box via a cable connection to a mobo USB2 header. But some Controllers come with hand-held boxes with buttons for you to make manual settings, and some come with none of that and you must use buttons on the controller box itself.
To control the speed of older 3-pin fans a header must supply the fan with a VOLTAGE that varies from 12 VDC (full speed) down to about 5 VDC for min speed without stalling. The newer PWM type fan with 4 pins instead always receives the full 12 VDC power supply but also receives the new PWM control signal from Pin #4. It has a special chip that uses that signal to modify flow of current from that power source through its windings to alter speed. An older 3-pin fan connected to that set of signals "sees" a constant 12 VDC power supply and has no way to use a PWM control signal, so it always runs full speed. A normal HUB merely takes the host header's PWM signal and shares it out to all its fans, so a 3-pin fan cannot be controlled that way.
A fan generates a speed signal (5 VDC pulses, 2 pulses per revolution) and sends it back to the host header on Pin #3 for counting. The header can show you that for interest, and does use that to monitor the fan for possible failure (no speed signal). But it does NOT need or use that for controlling speed. In fact, the automatic speed control system on a mobo header only cares about the TEMPERATURE at a sensor (either inside the CPU chip or on the mobo) and it manipulates the fan speed to keep the temp on target. It really is a TEMPERATURE Control System - it does not focus on controlling fan SPEED.
A fan with only a 4-pin Molex connection to the PSU draws its power from two lines of that connection (+12 VDC and Ground), and has no way to send a speed signal back to anything. So its speed cannot be "seen" anywhere, and cannot be controlled from a power supply that never changes. BUT if you re-wire it to power it from a mobo header AND that header is using the older method of speed control for 3-pin fans (called Voltage Control Mode or DC Mode), then its speed CAN be controlled that way, even though it still cannot send any speed signal out.
Because the two fan types require different types of control signals you cannot mix them on one fan header easily. In your case, though, you CAN get a mixture of both fan types to work for case ventilation with the only SYS_FAN header your mobo has. The mobo manual indicates that header only functions in the new 4-pin PWM Mode. You need to buy a particular Hub, the Phanteks Universal Fan Controller model PH-PWHUB_02
https://www.amazon.com/Phanteks-Universal-Fan-Controller-PH-PWHUB_02/dp/B07NHQRCRM/ref=sr_1_1?crid=F4NT406IWGL5&keywords=phanteks+universal+fan+controller&qid=1661347461&sprefix=phanteks+universal+fan+controller,aps,101&sr=8-1
It can work with input signals from a mobo header of either type - in your case, it will be the PWM type. You connect a line from the SYS_FAN header to this Hub's input port, and a cable from the Hub to a mobo SATA power output connector. It has two different groups of output headers for fans. Four are for 4-pin fans only and use the PWM signal system. Three are for 3-pin fans, and the Hub "translates" the PWM signal from the mobo header into Voltage Control Mode signals for these fans. Port #1 can work with either fan type but you most plug a fan in there - it is the only output port that will send its speed signal back to the mobo header for counting.
Using this Hub you CAN let the mobo SYS_FAN header control the speeds of all your case ventilation fans according to the temperature measured by the mobo sensor. Just plug each fan into the correct type of Hub port.
Now, how to modify your fan(s) with a wide Molex connector. I fully expect that the connector has Black and Red wires only. Further, when you plug that into a Molex output from the PSU you will see that the Black wire mates with one of the two middle Blacks from the PSU, and the Red wire mates with a Yellow from the PSU. Now look at the connector on the one 3-pin fan you have already. Note that ONE side of it has ridges down the side. The wires will be Black on one end, Red in the middle, and Yellow on the other end. On the 3-pin fan's connector, Black = Ground, Red = +VDC power supply, and Yellow = Speed pulse signal. For the fan(s) with a Molex on its wires, you need to get some 3-pin female fan connectors - either salvaged from an old non-functioning fan or by buying some 3-pin fan extension cords. Cut off the Molex from a fan and one end of the replacement connector, and just splice wires together matching Red to Red, Black to Black. Tape the end of any Yellow on the new connector/wires so it does not short out and just curl it up neatly - do not connect to anything on the fan. Now you have a standard 3-pin fan whose speed CAN be controlled by varying voltage from the Hub port, but without any ability to send a speed signal back. Ta-da!
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