IIRC correctly, 3 continent fares are only possible from North America, Asia, Europe with a routing involving these three continents.

If you include South America, Africa or SWP you need at least 4 continents.

Asia-Africa-North America is not possible because there's no direct flight between Africa and NA (you have to include Europe somewhere). South America-Africa-SWP is not possible because there's no direct flight SA-Africa and Africa-SWP. Europe-South America-Asia is not possible because there's no direct flight SA-Asia (JL used to fly GRU-NRT but via JFK). And so on.

OW does not even sell 3-continent fares from the southern hemisphere because you cannot construct a valid 3-continent itinerary from there.