Given three knights that can move to d2, one on b1, one on b3 and one on f3, how would you notate knight on b3 moves to d2? N3d2 is ambiguous, so would it be correct to write Nb3d2? Presume that a player has underpromoted to a third knight in this example.
Yes. You should disambiguate by adding either
- the file of departure (if they differ); or
- the rank of departure (if the files are the same but the ranks differ); or
- both the file and rank of departure (if neither alone is sufficient to identify the piece – which occurs only in rare cases where a player has three or more identical pieces able to reach the same square, as a result of one or more pawns having promoted).
This is an example of 3: neither rank or file alone is sufficient to identify the piece.