I don't remember when 3 finger drag doesn't work on my Mac. I used 3 finger drag to move application window, but this does't work suddenly, the day is ambiguous but maybe changed my Mac to EI Capitan from Mavericks.
I found the reason why 3 finger drag doesn't work because this gesture assigned Mission Control and App Expose as default. The following procedures can enable 3 finger drag again.
- Open System Preferences
- Open Accessibility
- Select Mouse & Trackpad
- Open Trackpad Options...
- Check "Enable dragging" and select "three finger drag" in the right pull down menu of "Enable dragging"
Enabling "Enable dragging" and selecting "three finger drag" makes gesture both "Mission Control" and "App Expose" changed using 4 fingers.