I gather you would like to have the actual CPU cooler be run from the CPU_FAN header but have your CASE ventilation fans run from another header. That is the right way. The problem is that ALL your fans are 4-pin PWM type. There is an ideal solution for you.
First, let me distinguish between a SPLITTER and a HUB for fans. A SPLITTER is a simple device that basically connects all its fans in parallel to the host header, and ALL fan power is drawn only from that header. Now, most mobo headers can supply power at up to 1.0 A max current to the total load, so you CAN use a Splitter to connect two or more fans to one header as long as you find (from makers' specs on the fan or web page) the max current for EACH fan and add them up to be sure the total does not exceed 1.0 A. And you CAN power and control the speed of 4-pin PWM fans from a 3-pin older mobo header that uses Voltage Control Mode - it is not technically the ideal way to run such a fan, but it works. A SPLITTER has one input arm that plugs into a mobo header, and two or more outputs with male connectors for plugging in fans. Its has NO other connection arms.
A HUB is a different device although some look a lot like a Splitter. A Hub gets its control signal for speed of its fans from the mobo header. Almost ALL Hubs are designed to get the PWM control signal form the header's Pin #4, so they can ONLY be used with a header of the 4-pin type, and ONLY with 4-pin fans. The Hub has one input "arm" and several output male connectors or ports, PLUS one extra "arm" that must plug into a power output (typically SATA type) from the PSU. ALL power for its fans is drawn from the PSU so the current limit of the host header does NOT apply.
Sellers often use the terms Splitter and Hub interchangeably and that is wrong I say. WATCH for the presence of a third "arm" that connects to the PSU. No arm like that, it's a Splitter; WITH that extra "arm" it is a Hub.
The ideal solution for you is a particular HUB with an unusual design - the Phanteks Universal Fan Controller
https://www.amazon.com/Phanteks-Universal-Fan-Controller-PH-PWHUB_02/dp/B07NHQRCRM/ref=sr_1_1?crid=3PJ2UZUBG7PCC&keywords=phanteks+universal+fan+controller&qid=1657077580&sprefix=Phanteks+Universal,aps,81&sr=8-1
It is unique in two ways. First, it has separate groups of output ports for 4-pin and 3-pin fans. (Plus, Port #1 can adapt itself automatically to either fan type). For each group it sends out the ideal set of signals for that fan design. Secondly, its input circuitry also automatically detects and adapts to the type of signal coming in from the host mobo header. Then it "translates" that speed information into the correct forms for 3-pin fans on the 3-pin ports, and for 4-pin fans on the 4-pin ports. This last feature is quite unique among Hubs. In your case, you can connect one of these to either of your 3-pin CHA_FAN headers and them plug your 4-pin fans into its 4-pin ports, and you WILL get full control of fan speeds using the optimal PWM signal form those fans need. Just be sure to plug one of those case fans into Port #1. On this Hub like any other, ONLY port #1 will send its fan's speed signal back to the mobo, and the speeds of its other fans are reported nowhere.
Using that Hub you can use many 4-pin fans (their power source is the PSU) all with speed controlled by a mobo 3-pin CHA_FAN header.
A last hint. Do not try to use the mobo's PWR_FAN header. That is a special-purpose header used over a decade ago intended for special signals input from the PSU directly via a pair of wires. It could read the speed of the fan inside the PSU and display that for you. But most such headers do NOT have any way to CONTROL a fan speed, so they can't be used as normal fan headers.
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