I think (correct me ) Lucas was the first to create the trilogies in hollywood. nowadays it's just bread and butter for the franchises, ...Batman, Ironman etc.
I'd compare the Apu Trilogy to great works of literature like David Copperfield or Great Expectations. I can't think of any film trilogies to compare it to.
No, other trilogies were created previously. For example, Frankenstein (1931) with followups Bride of Frankenstein (1935), & Son of Frankenstein (1939) all starring Boris Karloff. Also, depending upon how you view things, there were many series movies created throughout the years with 3+ films. An example is the Thin Man Series: The Thin Man (1934), After the Thin Man (1936), Return of the Thin Man (1939), Shadow of the Thin Man (1941), The Thin Man Goes Home (1945), and finally Song of the Thin Man (1947). These 6 films (first 3 are great, other 3 are quite lacking) could easily constitute a set of trilogies as they all star William Powell and Myrna Loy.
But were those trilogies intended to be trilogies from the get-go, or did they just keep making movies and eventually stopped at 3? I think with Apu, it was always intended that there would be three films.