Source and Destination IP of TCP connection

Five Tuple Identifier of TCP connection is (TCP, local IP, local port, remote IP, remote port).

I have made 3 computers in virtual box and set BOX1 for client, BOX2 for median, and BOX3 for server. And made interfaces between them to communicate.

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So I have executed the server program on BOX3 and client program in BOX1. The client program calls

When I captured packets from BOX1 and BOX3, the IP Source and Destination IP address was like this.

BOX1's SYN Src : Dest :  BOX3's SYN Src : Dest : 

What I have thought is that in BOX1, Src must be and Dest must be And in BOX3, it must be for Src and for Dest. Because BOX1 communicates with BOX2 and BOX3 commuicates with BOX2.

Why does IP is like that?

Thanks for your help.