Is it possible to use my PS3 controller on my PC version of the Witcher 3?

If so, how?

Oh yeah, this is under Windows 8.1 ( for now ).

I'm surprised there isn't a sticky for using this (god awful) controller for the game. I did see someone say that it was supported, but that could of been PS4.

The DualShock3 is generally not well supported on Windows.

Generally, the best way to use it for gaming on PC is to use an input wrapper to emulate an Xbox controller. It's a one time install, generally bug free and very customizable.

You can find the instructions here:
http://forums.pcsx2.net/Thread-XInput-Wrapper-for-DS3-and-Play-com-USB-Dual-DS2-Controller
note: If you are using a wired controller just connect your controller instead of connecting the dongle when instructed to do so.

P.S. I am saying generally because I have heard that there is direct support for the DualShcok3 for TW3 but I have no way to confirm that.