BigDownload.com has posted an impressively knowledgeable and informative Diablo 3 preview. The first page is just a space filler, with only one paragraph of introductory comments, but page two on Background and page three on Hype are both well done. Here’s a quote from page two, which efficiently summarizes the plot of D1 and D2, and then segues into a discussion of the game’s mythological elements.
The three Prime Evils in Diablo are named after three representations of demonic figures in Judeo-christian mythology. The first, Diablo, is the Spanish word for devil, which makes the reference there very simple and straightforward. Baal is a term used throughout the Old Testament to refer to any god that was not Yahweh. It was used in a derogatory manner, and its connotation with evil, destruction, and falsehood has led it to be used in several settings to refer to an evil deity or demon. Mephisto is more recent, with origins in the classic myth of Faustus, a man whose interactions with devils put his soul on the line. Baal (as Bhaal) is notable in that it is the name of a deity in the Forgotten Realms setting, which spawned the classic Baldur’s Gate RPGs (they revolve around his influence).