I decided to upload some parts of my schematics which might be useful for diy projects. 

Simplified schematic of a 4-pin (PWM) to 3-pin Adapter:

For Q4 and Q6, i used a double transistor (MBT3904). It should work with discrete transistors aswell.

M4 might need to be attached to some kind of heatsink, depending on how much current your fan will draw. 
The circuit itself is a simplified Low-Dropout-Regulator (LDO). It may not be the cheapest solution to do it this way, but i just liked to have a solution that can be built with discrete components.