Two "Trilogies"

Isaac Asimov wrote eight installments of a "Foundation" future history series. These were collected, with one new introductory story, in three volumes, hence the inaccurate phrase, "Foundation Trilogy." This "Trilogy" is neither one work in three parts, like JRR Tolkien's Lord Of The Rings, nor three related works, like James Blish's After Such Knowledge. Asimov later wrote four Foundation novels.

Poul Anderson wrote twenty installments of a "Psychotechnic" future history series. Sixteen of these were collected in three volumes, hence another "trilogy," although not, in this case, a complete collection of the series. Anderson later wrote several other future histories.

When the Psychotechnic History is considered as a unit, it can be seen to have remarkable parallels with the far better known Foundation series:

both Seldon's psychohistory and Valti's psychotechnics can be used to predict events and to direct society;

both are initially successful, with Seldon's Foundation defeating greater powers, including even the shrunken Empire, and the UN world government, advised by the Psychotechnic Institute, defeating nationalists, militarists and dictators;

however, Seldon was unable to predict a powerful individual mutant, the Mule, as psychotechnics is unable to prevent the social dissatisfaction caused by mass technological unemployment;

the Mule defeats the Foundation and the Humanist Revolution outlaws the Psychotechnic Institute;

however, the Foundation Trilogy ends with the psychohistorians on course to implement Seldon's Plan for a Second Galactic Empire while the Psychotechnic History ends with psychotechnicians coordinating Galactic Civilization long after the Fall of a First Empire.

So, to anyone who commends Foundation, Anderson fans can recommend the Psychotechnic History. This summary shows that Anderson addresses more plausible social issues.