3)

18...Ne5!

There are no prizes for other moves: 18...Bxe4?? is met by 19 Qxh6+ and 20 Qxg7#; 18...f6? 19 Bxf6! crashes through; 18...Kg8? 19 Nf6+ gxf6 20 Bxf6 gives White a killing attack; 18...Kh7?? 19 Qf5+ wins the bishop.

19 Nxd6?!

White goes wrong. The correct path was 19 Rd1 Bxe4 (19...c6 20 Nxd6 Qxd6 21 Bxe5 Qg6 might hold) 20 Qxe4 (20 Bxe5 Bh7) 20...Nc6 21 g4, which gives White attacking chances in return for the pawn.

19...Qxd6

19...cxd6 20 Rd1! is pleasant for White, but 19...Nd3+! 20 cxd3 Qxd6 is good for Black, and so a good alternative.

20 Bxe5 Qc6 21 Rd1 Be4! with counterplay and a secure position for Black.