The Forgotten Realms-licensed Baldur's Gate game series features a book called History of the Dead Three. It is worth 2 gp, so one can assume that it is not a very rare item. Hence, it should not be unreasonable to assume that at least sages and people who are experts in religious myths and stories should be familiar with the ascension of the Dead Three, at least as much as the games represent the setting. (The games were later converted to novels with some modifications, and FR novels are considered canon.)
It is worth noting that people are generally informed about religions in Faerun. For example in spite of the fact that Ao does not grant any spells nor influence the lives of people, a cult was still able to form around his "tenets" after the Time of Troubles. Another point to highlight is that Jergal's stepping down is a relatively recent event; he was still a greater deity at the time of Netheril.
Having said these, I feel it is best that I share some further data that might make the answer somewhat complicated. Jergal is an interesting entity, from his original conception at Ed Greenwood's gaming table, he was "a figure of mystery", quoting the Hooded One, Greenwood's official spokesperson on the Candlekeep forums. According to a 2015 invited blog post by George Krashos on Greenwood's now defunct Forgotten Realms Secretariat website, he was of a race of beings known as the spell weavers at the time of the sarrukh (-30000DR), and later achieved divinity somewhat accidentally. From that blog post we also learn that his relation to at least one of the Dead Three goes back further than what most experts know.
Note: The text that a player reads in the book from the computer game is identical to the text given in a sidebar on page 37 of the AD&D 2e sourcebook Faiths and Avatars. The same text is repeated on page 31 of the D&D 5e sourcebook Sword Coast Adventurer's Guide.