If I understand correctly, 3 finger drag does not equal to 3 finger gestures. When you do a 3 finger drag the pointer moves with your fingers, but when you do a 3 finger gesture the pointer remains in the same place.
I have done some modifications to the Synaptics Xorg driver to add support for 3-finger drag in Xubuntu:
This is a video showing how it works ...
And unfortunately you have to compile it from source ...
Note: this is only for my personal test, it comes without any warranty.
I have tested it only on a Magic Trackpad, but I think it should also work with MacBook's trackpad.