Right.
It only feels that way to Batman fans because they're getting more exposure to the character in other forms of media. We had Heath Ledger's Joker, Joker was one of the main antagonists in all the Arkham games, we had Jared Leto's Joker, we had whatever the **** the Gotham character was supposed to be, we just had Joaquin Phoenix's Joker, and there are all the Joker-centric storylines going on in the comic books.
Seems like a lot, but not everybody is playing the video games. Not everybody watched Gotham. Not everybody is reading the comic books. If you dilute it down to just movies, three appearances in the span on 11 years is not awful.