Question
You want to know discretely what the third way of the three-way handshake is in the context of TCP/IP networking. What is it?
Short Answer
It is the sending of a TCP packet with a flag of "ACK" from the client to the server/destination.
Longer Answer
The third way is sending a TCP packet with the "ACK" flag (of which several potential flags are possible) to the destination:
Client -----> Server
TCP is used in HTTP communications. TCP uses a three-way handshake; each way has a specific flag in a TCP packet and a specific source and destination (among the two end point devices involved). When a workstation tries to connect to a website (e.g., http://www.continualintegration.com), the workstation sends a TCP packet with a SYN flag to www.continualintegration.com. Then the server sends a SYN-ACK to the client, and this is the second way. Then the client returns with a TCP packet with an ACK flag to the server, and this is the third way.
To read more, see these:
https://ddos-guard.net/en/terminology/protocols/tcp-3-way-handshake
https://www.techopedia.com/definition/10339/three-way-handshake
What Is The First Way of The Three-Way Handshake?
What Is The Second Way of The Three-Way Handshake?