What is the third step in the three-way handshake?

FIN is part of the TCP four-way connection termination process used to close an established session, not part of the three-way handshake that opens one.

The ACK packet is the third step of the TCP three-way handshake, sent by the initiating client to acknowledge the server's SYN-ACK response. This final acknowledgment confirms both sides have synchronized sequence numbers and are ready for data transfer, formally completing the connection establishment before any application data is exchanged.

SYN-ACK is the second step of the handshake, sent by the server to acknowledge the client's SYN and simultaneously synchronize its own sequence number.

SYN is the first step of the handshake, sent by the client to initiate a TCP connection by proposing an initial sequence number.