I have two 120mm fans in the front of my machine and I'd like to connect them as chassis fans, but I'd like them to share the same 4 pin connector on my motherboard so that they both scale up and down simultaneously. Is there a way to do this? Do I need a given adapter? Both fans are 3 pins, but I have 4 pin and Molex adapters for them. How can I make this happen?
You can totally do this.
There are 4-pin fan splitters like this one and 3-pin splitters like this one, that will let you power multiple fans off of one motherboard header. I'm using similar splitters (three-way instead of two-way) on a few of my systems at home.
I'm not sure if it's the case with the linked 4-pin splitter, but most of them only provide the fourth pin to one of the split connectors, as providing it to all of the fans would easily confuse the motherboard's ability to detect and control speed.