For a while, when I was playing techno btw. 2000-2002 I was quite a serious 3 deck user. I would put on 3 tracks at the same time, or mix in two and drop the third. But that was pretty tracky/loopy techno. I started working towards playing more songs, and working to present those songs - and 2 records became more the way I did it.
In a bigger club, there would always be 3 decks even in the 90's, not because the djs would be mixing 3 decks all the time, but because they would have the ability to cue something else up, or get ready for a quick transition. You might just want to play 30 second intro/noise off of one record, and then you could already have the next thing cued up and ready to go.
But I started djing in 1992 - so I am a bit of a dinosaur at this point.
I like three decks for mixing. I always have had a mixer with at least three channels and that was just the way I mixed. not always playing three things at once but having something ready on deck 3 while I work on beat matching decks 1 and 2. The vocal track was essentially deck 3. that's the way I've been practicing for about 10 years, not quite a dinosaur, but not a rookie either.