I'm just testing the X3 using a loan unit from HP. I can confirm on the device I have, HP's Pro Tablet 608 pen works on the X3. I have a Pro Tablet 608 that came with the active pen and it works!
At this stage, the pen only acts as a basic point and click device. There's no handwriting input that I can find in the X3 itself and OneNote won't let me draw.