It would save you a lot of money to just crimp or solder something like that yourself. However it would be complicated.
Have you tried or considered 3-pin "extender" PCB boards at all? They are low profile enough to fit behind the motherboard tray yet still function the same way a splitter cable would.
Here's an example of a splitter with a molex end: LINK
If you need to maintain the RPM like for something like a fan controller, here's a three pin version (these are sold in different sizes up to 5 connections, this is just the link to the 3x one): LINK