Lock On: Flaming Cliffs 3 (FC3) is the next evolution of the Flaming Cliffs series. FC3 features the F-15C, A-10A, Su-27, Su-33, MiG-29A, MiG-29S, Su-25T, and Su-25. FC3 aircraft provide an easy learning curve for new players and focuses on a broad range of aircraft rather than a detailed single aircraft. FC3 adds a number of new features and improvements to previous versions of the Flaming Cliffs series. FC3 is a module of DCS: World, which makes it compatible with all other DCS: World titles like A-10C, Black Shark 2, P-51D, and Combined Arms.
Source: The Game Website.
So at first Lock On: Flaming Cliffs 3 was a stand alone game, the third in the Lock On series.
Then DCS World was created: a sort of sandbox simulator world for which you could purchase different real-to-life vehicles (mostly planes) to play campaigns in. FC3 then was added to that world, as a whole, to the DCS-World. So you had a number of different planes, which, though complicated to fly for most players, were not real-to-life simulations but were sold as "focused on ease of use without complicated cockpit interaction, significantly reducing the learning curve". More about the action than all the nuts and bolts, so to say, though 'ease of use' may be in the eye of the beholder.
At some point ED also started to sell all FC3 planes (F-15C, A-10A, Su-27, Su-33, MiG-29, and Su-25) separately for DCS-World with a warning for FC3-owners: "Note: the aircraft model is identical to the Flaming Cliffs 3 module. Owners of FC3 do not need to purchase."