As far as I know, prior to ESO we didn't know the Ebonheart Pact or Daggerfall Covenant existed at all, and the only reason we knew about the Aldmeri Dominion is because the version running around in Skyrim is Dominion #3 or #4. (Version #2 was around in Tiber Septim's time.)
"And it shalt be known that Alliance with thy color of sky shalt solely fight Alliance by the color of Sun and that Alliance with thy color of tomathoe shalt solely fight Alliance with thy color of Sun.
For so strong and noble the Alliance with thy color of Sun is that no other measures will hence forth grant the Alliance with thy color of sky or Alliance with thy color of tomathoe victory."
But that's just the legend.......
Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium dolorem que laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur, vel illum qui dolorem eum fugiat quo voluptas nulla pariatur?
The Second Era Aldmeri Dominion was mentioned in 'The Pocket Guide to the Empire' booklet packaged with Oblivion. In the Chapter about the 2nd Era, after the death of the last Potentate, it mentions the Knahaten Plague and the Akaviri Invasion of 2E 572 (no mention of the Ebonheart Pact, just that the Akaviri were defeated after attacking Morrowind). Then it goes on to say:
"The first sign of reunification occurred in the west. The Altmer of Summerset, long concerned with their own wars with other island kingdoms, allied with Valenwood to form the Aldmeri Dominion for their common good."
No mention of the Khajiit being involved, but as it immediately skips ahead to the rise of Talos, a lot is left out. It also mentions the constant warfare for the Imperial throne, but no specific mention of the Three Banners.
As this book was published in 2006, this is the earliest mention I've found relating to any of the ESO factions.
"Ours is not to reason why, ours is just to blame them afterward." -- Bob Varsha