Those would be A Darkness Gathering, Masters of Eternal Night, and Dawn of the Overmind, the three "Monstrous Arcana" adventure tie-ins with Bruce R. Cordell's Second Edition AD&D accessory, The Illithiad. (If memory serves, I think Bruce wrote the three adventures as well.)
Right. There's also similar series for Beholders and Sahuagin (sourcebook followed by 3 adventures).
Something that might be important to you:
For an unspecified time, all the adventures (but NOT the sourcebooks) of those three series (Illithids, Sahuagin, Beholders) are on sale for $3.99 (down from $4.74) at svgames.com.
If I hadn't run out of money--if I hadn't run myself into credit card debt this month, so much that I don't think I'll pay it off all at one time--I would have bought them, as well as the Birthright player's guides to domains and a bunch of other stuff...