Africa is enough separate that you can make an argument that it isn't part of Asia+Europe. I can see no good reason to call Europe and Asia a separate continent. (I'm not sure if it is bigger than north+south America - it feels like it is, but maps can be deceiving and can't be bothered to look it up). Of course if Africa is not part of Asia+Europe then it is hard to argue America is one continent.

In the end we don't have a good definition of continent though, so you can find plenty of weird things you can argue should be the definition and thus come up with a lot of contradictory answers.