Question can i adjust speed of 3 pin fans with fan multiplier on b450m s2h?

Paperdoc

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That item you have linked to will NOT do what you want. Although it looks like a simple connection of wires in "arms", it really is a fan HUB. How can you tell? Well, the key identifier for a HUB is that it has one cable or "arm" that must plug into a power outlet from the PSU for power to its fans. That item has a connector that plugs into a 4-pin Molex output from the PSU. What a HUB does is distribute the power from the PSU to its fans, plus the PWM control signal from a mobo fan header, so that the fans themselves can use those to control their speeds. BUT that means that ALL of the fans used MUST be the new 4-pin PWM type, which you do not have.

What you need is called a SPLITTER, but be wary of on-line listings. Many sellers use the two terms - Splitter and Hub - as if they mean the same thing, and they are quite different. A Splitter has one "arm" or cable that plugs into a mobo fan header to get power for all its fans, and that power is governed by the header (it regulates the voltage supplied to the fans) to change fan speeds. Then all its other "arms" end in male (with pins) connectors to plug your fans into. It may have only 2 outputs, or more - you would need one with at least three. The Splitter has NO connection to a PSU output. You can use a Splitter made for either 3-pin or 4-pin fans - the connectors are designed to work with either fan type. When you plug one of your 3-pin fans into an output from a 4-pin Splitter, it just does not use Pin #4. Here's an example of a 2-pack of 4-pin Splitters

https://www.amazon.com/Splitter-Computer-Extension-Converter-TeamProfitcom/dp/B07F8LV1BY/ref=sr_1_3?dchild=1&keywords=fan+splitter&qid=1602685651&sr=8-3

In one of the photos close up of the outputs, is shows that one has all 4 of its pins, and the others are missing Pin #3. (They use the labels Master and Slave, and that is not right - ignore that). This is the correct design. A mobo fan header can deal with the speed signal sent back to it from only ONE fan. So a Splitter will send back one such signal via Pin #3, and ignore the speeds of all its other fans by not having that pin in a connector. This Splitter also looks like a collection of wires in "arms" like the one you linked, but it does NOT have a connection to the PSU. Note that some Splitters can look like little circuit boards, but still have the same connections available.

There is a limit when using a Splitter. All of the fan power comes from the mobo header, and almost all headers can supply up to 1.0 A max current. So you need to know the fan's max current load specification, and you add up all of those for the fans on one header. Today most computer fans pull from 0.1 to 0.25 A, so three is quite OK. The only ones that can be trouble are ones called LED Fans. These came before the popular RGB fans. LED fans have some LED lights in the frame of only one colour, and they have only one wire cable from the motor that supplies power for both the fan motor and the lights. (Today's RGB fans have a second separate cable for light power.) LED fans often pull 0.3 to 0.4 A per fan because of the added light power. So IF you have that type, check the power consumption specs carefully.

When you power 3-pin fans from a mobo header, the header MUST be set properly. In BIOS Setup for that header, ensure that it is set to use Voltage Control Mode (aka DC Mode), and not PWM Mode.

If you cannot find a Splitter with three outputs, get two of the common 2-output ones. Plug the second one into one output of the first to make a small "stack" that produces three outputs from one mobo header.
 

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