The same rule applies to Africa. It is not possible to have a 3 continent ticket when Africa or South West Pacific is used. The reason for this is that there are no OW operated flights between the Americas and Africa, thus no way to complete a 3 continent round the world trip. It is also the reason why you can enter some continents twice, provided one entry is a transfer without stopover.

It had been hoped that when MH joined, 3 continent trips using Africa/SWP could be possible, but MH recently pulled the route that would have allowed that.