Hi burtus - Did you have any luck in either ID'ing the 3 pin connector, or in otherwise getting ahold of this part via aftermarket?
I personally have seen the part shipping from china->AUS for approx. $20 AUD, with maybe a month shipping est.
My PC shop didn't connect the fan control on build, and didn't supply us with spares from the build ().
Re. your specific request:
USB2 9 pin headers are well documented (link - every example I've seen appears to connect [4, 6, 8], but that's likely interchangable with [3,5,7]), & I've found multiple pictures that indicate the pin positions on the 3 pin side, however I don't know the form factor of the 3 pin-side connector.
If you've found any pic of the 3 pin side, you'll notice black, white and green. These have well-documented meanings [black = ground, white = data+, green = data- ], and on most postings you can actually confirm that against the USB header side.
I'd be surprised if the 3 pin connector was not some standard form factor, and from squinting at blurry pictures, I have a few suspicions of what that may be, but nothing solid.
I've found a cable-id request thread (reddit link) with HQ photos of a very similar looking part, that people suspect is the same cable.
The poster says it's "just over 4mm wide"