I had this issue when building my last PC and found the internet to be relatively unhelpful actually. If this post saves one person then I’ve done my good deed for the day!

The long and short of it, is that you need to buy a product for circa £20 (summer 2025 pricing). You can see the one I selected in the video below. I’ve started the video at the timestamp of the unboxing, so you can see it for yourself. It is a black box that is circa 10mm x 50mm x 30mm. 

This has 3 no inputs and 1 no output. 1 input from your PC Case reset switch, that allows you to cycle through the modes. 1 input from a sata power cable from your PSU. 1 input from your 3 pin RGB. 1 output that is the 4 pin RGB header.

The box that connects everything has preset RGB modes. These will link with the input source (e.g. your 3 pin RGB fan) and then display patterns, based on the LEDs in the fan and the order those are wired up etc. There is an option to switch to ‘motherboard control’ but I don’t care enough to stomach the bloatware. Furthering this, don’t worry if you don’t want to hook up your reset switch, there are buttons on the box itself that allow for mode and speed selection.