Launcher3 is the default AOSP launcher. If you build Android from source, you'll be able to build Launcher3 as the default system launcher (which is required). Launcher3 is also the framework from which most Android launchers are built upon. Nova, Action Launcher, Google's Now and Pixel launcher...
Now, the 'Google Now Launcher' app that you have installed is really more like a stub that enables the Google App to function as your launcher, and it is not dependant on the existence of Launcher3 (though, if we could look at the complete course for the Google Now Launcher, you would see Launcher3 code in it). With the Google Now Launcher, they basically took Launcher3, then stuffed it inside the Google App so there is tight integration with the feed and On Tap... So other than the Google search stuff, its pretty much Launcher3.
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