That gives black the chance to convert the pawn at b2 to a queen by pushing it to b1.
If white chooses to take that fresh queen, black snaps back with the rook at b8, checking white’s king.
If white ignores the freshly converted black queen and moves queen to g6, the converted black queen can diagonally fall back to g6, supported by the pawn at h7, blocking check.
No mate in 3 that way.
