I’d guess that maybe these three became farmers after their racing careers. It is certainly true of Brabham and Scheckter. However, wikipedia has no info concerning Farina’s agricultural exploits, nor does it shed any light on whether any champions since Scheckter have run a farm.
Purely because he’s Australian (they tend to own lots of land, right?), is a former champion, and won his championship after Scheckter, I’ll guess Alan Jones.