I am a Ferguson fan as much as anybody on this board.
I still believe Harry Ferguson did not invent the three point linkage. His various patents refer to such things as "converging" three point linkage, direct hitching, weight transfer, depth control, anti-sway blocks for three-point hitch, etc.
I have seen a three-point hitch on an early 1920's tractor (not a Ferguson). The tractor had a three point hitch that pulled and raised a rear- mounted plow. There was a plow attached to the tractor. The plow was raised with a mechanical lift. This tractor did not have "converging" three-point linkage, depth control, weight transfer, anti-sway blocks. Neverless, it had a three-point system. The plow was attached to the tractor at three points. Two points pulled the plow; one point raised the plow. It was not a sophisticated system, but it had three points. Therefore, Harry Ferguson did not invent the three-point system.
Harry Ferguson worked extensively with tractors in the early part of the century. He was a dealer for the Overtime Tractor company (known as the Waterloo Boy in the United States, the forerunner of John Deere). There were many brands of tractors with rear-mounted plows at that time, one as early as 1911. Ferguson was commissioned by the Irish government to improve farming systems. He had to have been aware of these other linkage systems.
Harry had a lot of inventions, and I agree he improved the three-point system with draft control and made it a workable system. The three point system was not new. What was new was "converging" three point lingage, draft control, rigid top link.
A patent does not have to be a brand new idea. Changes or improvements in an existing system can get you a patent. Harry Ferguson improved the three point system; he didn't invent it.