Complete solution including diagrams (congrats @iceblue for spotting it):

1. Nd6! Idea: 2. Nf7# checkmate.

Black has now several options. I will consider:

a)

1. ... Nxd6
2. Ba5 And whatever black plays, he will get checkmated by 3. Bxc7#. For example 2. ... Reg8
3. Bxc7# Checkmate.

b)

1. ... cxd6
2. Qc2! Idea: Qxc8# checckmate. 2. ... Nc5
3. Ba5# Checkmate.

c)

1. ... Ref8
2. Qe8+! Rxe8
3. Nf7# Checkmate (smothered mate).

d)