I'm super excited to have found this thread because I'm trying to solve this exact problem! In fact I'm building a small PCB that will take in a 4-pin 12v RGB and spit out a 5v 3-pin, it'll also include a fan splitter (since I'm using the Phanteks Halos).
Your schematic is extremely helpful @nicegamer7 but it doesn't answer the question I've been struggling with. Where the hell do I hook up the ground signal? All the research I found always had a common ground pin with three 12V PWM signals. It's easy since you can just chain all the ground signals together. But because there's a common power pin here, where does the ground go?
In my case, I have written some software for the ATTiny85 that will smooth out a PWM signal and convert it to the 3-pin LED strip. In my case I'm guessing the frequency of the motherboard PWM is 25Khz? so I'm using an RC circuit to convert each color channel to a voltage which I feed into the ATTiny (though I need to convert it to 5V somehow as well). But my problem still stands, I have no idea where the ground goes.
