Is the Oedipus trilogy actually a trilogy?

The term "Oedipus trilogy" is actually incorrect. In antiquity, a trilogy was a group of plays performed together at a festival. The standard format was three tragedies followed by a satyr play. The three tragedies could either be on related topics, forming a true "trilogy" (e.g. Aeschylus' Oresteia) or on unrelated themes. Sophocles' Theban plays were written and performed at different times and thus do not constitute a trilogy, despite being thematically linked.