Installing the latest WinTab drivers fixed pressure for my Surface Pro 4. I had to disable pen & touch flicks after that though.

Not sure if this will apply to the Pro 3, but I just got the Pro 4 and installing the latest WinTab drivers got pen pressure working flawlessly in Painstorm. Just remember to disable flicks/gestures in Pen & Touch settings as it seems to interfere with making strokes

Since I updated to version 1.54, my Monoprice tablet no longer works in Paintstorm. I have tried running it as administrator, re-installing completely(deleting the preferences), unplugging the tablet and nothing helps. The tablet still works in Photoshop.