Batman sat on the Mobius Chair in Justice League #42,he asked it two questions: Who killed his parents with the response: Joe Chill, and the comic ended with the second question, what is the Joker’s real name? and Batman responded, “That’s not possible.”
So what response did he got from the chair?
In Justice League #50, we learn that when Batman asked (in Justice League #42) what’s the Joker’s true name or Identity
And in issue #50 we got to know that the Mobius Chair told him “there were three.” But three what? Three Jokers?
Turns out, the Mobius Chair likes to speak in cryptic riddles on occasion. (Then again, maybe the Chair was being literal and Batman should start looking for a clue, or something.)
Maybe the DC want’s to show that Joker has three personalities that have separate names to identify themselves
First is the name he was born with.
Second is the Joker – his public persona or stage name,and
Third is the name of the identity that embodies the impulses that drive him,like in the Batman Animated Series – Harvey Dent’s other personality was “Big Harv”.