The first business airplane to be CATIII qualified was not a jet...it was a Gulfstream One that belonged to the radio/television personality Arthur Godfrey.
It was equipped with dual Collins para visual displays (PVD) that was very similar in operation to the one fitted in HS.121 Tridents, and allowed CATIIIA hand-flown ops.
Godfrey used to talk about it extensively on his radio program in the early 1960's.
Other airplanes were regularly CATII equipped.
For example, I used to fly, many years ago a Fairchild FH-227B that was so equipped, and it worked to perfection.
Dual FD109 flight directors and dual radio altimeters...expensive, but when the weather turned sour, very useful.