Longest forced win after 3 moves It is possible to reach (exactly) #8 after just 3 moves:
[Title &quot;#8 after 3 moves&quot;]
[FEN &quot;&quot;]

1. e3 f6 2. Qf3 Kf7 3. Nh3 Qe8! 4. Ng5+ Kg6 5. Bd3+ Kxg5 6. Qf4+ Kh5 7. g4+ Kh4 8. Qg3+ Kg5 9. f4+ Kh6 10. g5+ fxg5 11. Qxg5#

Is there any longer forced win than #8 that can be reached (and proven) after just 3 moves?
This question is a variant of, and therefore not a duplicate, &quot;The quickest mate in n.&quot;
After playing around in the Lichess's analysis page for some time, I have found a mate in 10. Throwing the moves into chess.com's analysis page provides further confirmation. It has some wholly unique lines, and only a few dualed lines, making it reminiscent of a chess problem, per @Hauke Reddmann's above comment.
[FEN &quot;&quot;]
[Title &quot;#10 After 3 Moves&quot;]

1. e4 d5 2. h3 Kd7 3. Bb5+ Ke6 {Now the helping play is over.} 4. Qg4+ Kd6 5. e5+ Kc5 6. b4+ Kxb5 7. a4+ Kb6 8. a5+ Kc6 9. b5+ Kc5 10. Ba3+ Kxb5 11. Nc3+ Kc6 12. Qa4+ b5 13. Qxb5#