Your mobo has three CHA_FAN headers. See p. 1-14 of its general User Manual here
https://dlcdnets.asus.com/pub/ASUS/..._TUF_GAMING_X570-PLUS_WI-FI_UM_WEB_060319.pdf
So you could connect each case fan to its own separate header. The small advantage to that is the you can see the speed of each fan individually. (When you use a Splitter, only ONE of its fans gets its speed reported.) Then you can configure each header exactly the same for identical performance, or differently if you wish. Details of how to configure in BIOS Setup are on p. 38 of the BIOS Manual here
https://dlcdnets.asus.com/pub/ASUS/...ME_PRO_TUF_GAMING_X570_Series_BIOS_EM_WEB.pdf
You have not told us the full model name of those fans, but I can deduce from the RPM numbers you have the redux models. But further, do they have "PWM" on the end of their names or not? Older design fans have THREE wires from them ending in a connector with 3 holes. The newer design has FOUR wires and holes, and these are the 4-pin PWM fans.
In configuration options you need to set three items: MODE (the signal type used to control fan speed), temperature SOURCE (which temp sensor is used to guide it) and PROFILE (how the header decides what speed the fan should run).
MODE should be set to DC Mode for 3-pin fans, or PWM Mode for 4-pin fans.
Temperature SOURCE should be the general motherboard sensor for all your case fans.
PROFILE normally is set to "Standard" which will use a pre-set "fan curve" of speed versus temperature measured and automatically change the fans according to changes in heat removal required. Note that a FIXED speed arrangement can NOT give you ideal cooling under changing conditions. Also note that running a LOWER speed to make it quieter means you deprive your system of the cooling it normally needs.
You should be aware that fan SPEED is not what you really want. Fan AIR FLOW is what is important - that's what removes heat - but it is VERY hard to measure. Speed is easy so they tell you all about that. But if you look at the specs you won't be surprised to see that a larger fan blows more air than a smaller one running at the SAME speed. That is why those two fan sizes were designed with different max speeds. THEN recognize that the mobo headers do NOT care what speed the fan runs - they only manipulate the signal to the fans to ensure the TEMPERATURE at the sensor is on target. That signal sets (roughly) a "% of max power", and NOT a specified speed.
Go to p 10 and 11 of the BIOS Manual where it shows you the QFan Control screen. If you try out the PROFILE options across the bottom you will see they each provide a pre-set "fan curve". The LAST one ("Manual") allows YOU to set up that curve by dragging points to the speeds you want. You could set it to one speed for all temperatures, but you also COULD set it to speeds a bit LOWER that the default "Standard" curve. Just realize the quiet you get means your system has to run hotter.
Also in the General User Manual see p 3-18, heading 3.9.4 about Armoury Crate. This is a software utility on the CD with your mobo that you can install and run under Windows. Unlike the BIOS Setup system, this app is available at all times in normal Windows use and contains tools for MANY adjustments to your system, including how the fans and cooling are set.