"Force Touch" is both a generic industry term, and in a certain format, a trademark of a company called NextInput, which many believe licenses Apple with the Mac touchpad "Force Touch" sensor technology.
"3D Touch" seems to be an Apple invention.
Sourcing from patent information and Apple interviews, it appears that the primary difference is in the physical layout and assembly of the pressure sensors. Here, I've made a drawing:
Force Touch on a Mac touchpad or on the Apple Watch, uses sensors around the edge of a rigid piece of metal / glass. (On the Mac, I believe it's four sensors, one at each corner.)
3D Touch on the iPhone uses multiple transparent sensors to measure the force of presses on a piece of special flexible Corning Gorilla glass over the display. (On the iPhone, one patent talks about a grid of 8 x 12 (96) sensor locations.)
Although they both can measure a range of pressure, the 3D Touch layout allows more precision as to the touch point, and in theory even multiple pressure touch points.