user9192: It is possible to reach (exactly) #8 after just 3 moves:
[Title "#8 after 3 moves"]
[FEN ""]

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, "The quickest mate in n."

user15543: After playing around in Lichess's analysis page, I found a mate in 10. Throwing the moves into chess.com's analysis page provides further confirmation. It has a few wholly unique lines, and some dualed ones, making it reminiscent of a chess problem per @Hauke Reddmann's above comment.
[FEN ""]
[Title "#10 After 3 Moves"]

1. e4 d5 2. h3 Kd7 3. Bb5+ Ke6 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#

It can be done the other way around as well.
[Title "#10 After 2.5 Moves"]
[FEN ""]

1. d4 e5 2. Kd2 Bb4+ 3. Ke3 Qg5+ 4. Kd3 e4+ 5. Kc4 b5+ 6. Kxb4 a5+ 7. Kc3 b4+ 8. Kb3 a4+ 9. Kc4 Ba6+ 10. Kxb4 Nc6+ 11. Kc3 Qa5+ 12. b4 Qxb4#


9192: <p>It is possible to reach (exactly) #8 after just 3 moves:</p>
<pre><code>[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#
</code></pre>
<p>Is there any longer forced win than #8 that can be reached (and proven) after just 3 moves?</p>
<p>This question is a <em>variant</em> of, and therefore not a duplicate, <a href="https://chess.stackexchange.com/questions/34876/the-quickest-mate-in-n">&quot;The quickest mate in n.&quot;</a></p>
 15543: <p>After playing around in <a href="https://lichess.org/analysis#0" rel="nofollow noreferrer">Lichess's analysis page,</a> I found a <strong>mate in 10.</strong> Throwing the moves into <a href="https://www.chess.com/analysis" rel="nofollow noreferrer">chess.com's analysis page</a> provides further confirmation. It has a few wholly unique lines, and some dualed ones, making it reminiscent of a chess problem per @Hauke Reddmann's above comment.</p>
<pre><code>[FEN &quot;&quot;]
[Title &quot;#10 After 3 Moves&quot;]

1. e4 d5 2. h3 Kd7 3. Bb5+ Ke6 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#
</code></pre>
<p>It can be done the other way around as well.</p>
<pre><code>[Title &quot;#10 After 2.5 Moves&quot;]
[FEN &quot;&quot;]

1. d4 e5 2. Kd2 Bb4+ 3. Ke3 Qg5+ 4. Kd3 e4+ 5. Kc4 b5+ 6. Kxb4 a5+ 7. Kc3 b4+ 8. Kb3 a4+ 9. Kc4 Ba6+ 10. Kxb4 Nc6+ 11. Kc3 Qa5+ 12. b4 Qxb4#
</code></pre>
