Sorry, a little more complicated. You certainly CAN plug any 3-pin fan into any 4-pin header (mobo, Splitter or Hub) and they will work. BUT if they receive only the new PWM signals from a 4-pin header, a 3-pin fan will always run full speed.
3-pin fans can be speed-controlled ONLY by a header that varies the VOLTAGE fed to the fans. 4-pin fans work differently - they receive a constant 12 VDC power supply PLUS the added PWM signal from Pin #4, and they use their own internal chip to modify the power supplied to achieve speed control. 3-pin fans lack these parts and cannot use the PWM control signal.
The best common way to control several 3-pin fans from one mobo header is to use a Splitter. But watch out! Sellers use the labels "Splitter" and "Hub" as if they mean the same thing, but they are DIFFERENT. To me, a SPLITTER is a simple device that merely connects all its fans in parallel to the input signals, so all of the fans in this group get exactly the same voltage and draw all of their power from the mobo header. A Splitter has one "arm" that plugs into a mobo header, and two or more output "arms" for connecting fans, and NO other connections. It may look like a collection of cable arms, or a printed circuit board, or a box with ports in the side. A HUB, on the other hand, has those "arms" PLUS one additional that MUST plug into a PSU power output (SATA or 4-pin Molex) to get a source of constant 12 VDC power. All of its fans are connected to THAT power source and draw almost none from the mobo header. The HUB also distributes to its fans the PWM signal it gets from the header so all its fans can use that with their special chips. Thus a HUB can provide power to many fans (because it uses a high-power source). However, a SPLITTER is limited because its ONLY source is the mobo header, and almost all such headers have a limit of 1.0 A max current load.
When using a SPLTTER you MUST find the max current load spec for the fans you are connecting to ONE header and add them up. You cannot exceed that 1.0 A limit. But within that limit, the NUMBER of fans you can connect is not important.
Just to complete some general background, all fans generate a speed signal (5 VDC pulses, 2 pulses per revolution) that is sent back to the host header on Pin #3. Any mobo fan header can handle the speed signal from only ONE fan - more than that gets mixed signals that cause bad speed readings. So all proper Splitters and Hubs will send back to the host mobo header the speed of only ONE of their fans. In Some cases they mark the particular output port that WILL send that fan's speed back. In simple Splitters with several "arms", the most common method is NOT to include Pin #3 in the outputs of most "arms", so only one output connector has all 4 pins.
OP, look at the labels of the 3-pin fans you have. They should have a spec for the max Amps. Most use from 0.10 to 0.25 A max per fan. If you find that, add it up for the four you have not connected yet. Now, look at the fans already connected to mobo SYS_FAN or CHA_FAN headers. If you have only one fan per header, then you CAN use Splitters to connect several fans (often 3, maybe 4 - depends on the fan specs) to each of several headers. IF you have a MIXTURE of 3-pin and 4-pin fans, so NOT mix them together on one header/Splitter. See if you can arrange groups of 3-pin fans on several headers using Splitters like this 4-pin 3-arm model
https://www.amazon.com/Cable-Matter...1&keywords=fan+splitter&qid=1634703264&sr=8-4
or this 4-pin 5-output model
https://www.amazon.com/Splitter-Sle...1&keywords=fan+splitter&qid=1634703326&sr=8-8
Note that, although these are 4-pin designs, you CAN use them with 3-pin fans. For that last one like a circuit board, note that ONE of its outputs is labelled for use with the CPU cooler. Do NOT connect your CPU cooler to this. That one output is just the ONE that will send its fan's speed back to the host header, so be sure to plug one case fan in there.
Last step is configuring the mobo fan headers in BIOS Setup. Almost all these days have options to set them to use either Voltage Control Mode (aka DC Mode) or PWM Mode. When you connect 3-pin fans to a header, configure it for Voltage (DC) Mode; for 4-pin fans, set to PWM Mode. That sends the correct style of signals out to the fans.
If you need more exact guidance, tell us the maker and exact model number of your mobo. Then tell us the maker and model numbers of your fans - IF you have more than one fan model, tell us the count of each type. With that we can look up specs and advise.
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