D3X was used as a shorthand term for an "Diablo 3 expansion."
The term was created prior to the release any such product officially existing. At the time, Blizzard had frequently commented on their plans for at least two expansion packs for Diablo III, and since the Diablo II expansion, Lord of Destruction, was widely-known as D2X, this acronym for the Diablo III expansion seems inevitable.
The expansion pack eventually became Reaper of Souls.