Very nice.
I could only see #4, and would you believe, Gull 3, Komodo 8, and the latest version of Stockfish, all with 6-piece Syzygy tablebases are the same 
The second hardest ever mate in 3 puzzle
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Oct 4, 2014
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#2
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that is probably because its a mate in 4
I saw this puzzle on Quora, which is a general question-answer site ( not a chess site ). This was an answer to a question about hard chess puzzles. I ironically called it the 'second hardest ever' ( as opposed to another one, which I posted as 'hardest ever', it may not be, but it is reasonably hard: http://www.chess.com/forum/view/more-puzzles/the-hardest-mate-in-3-puzzle ). May be it is interesting what people at non chess sites think about chess.
white to move