p.boxed
	| Pawns start on the 7th rank. Move a knight to promote them.

h3 Divergences

p No castle, no en-passant capture.

p.
	...Only the initial position changes, but this makes a huge difference.
	In particular, castling would be rather pointless so it's disabled here.
	En-passant captures are impossible because all pawns already reached 7th rank.

h3 About the initial position

p.
	Since truly random start can allow a mate in 3 with a knight,
	the kings have at least one knight neighbor in the initial position.
	This allows to move free out of potential check from the very beginning.

p.
	A less constraining condition would be to require the two knights to stand on
	two squares of different colors, but it's not enough as proved by the
	following diagram.
	White can mate in 3: 1.Nc6 followed by Nb4 threatening both a2 and d3.

figure.diagram-container
	.diagram
		| fen:RBN1BRRQ/PPPPPPP/8/4n3/8/8/Nppppppp/brkbqr1n:
	figcaption After 1.Nc6 Nf3 2.Nb4 Ne5 (covers d3 but not a2) 3.Nxa2#

h3 Source

p
	| See for example the 
	a(href="https://www.chessvariants.com/diffsetup.dir/upside.html") Upside down chess
	|  page on chessvariants.com.