In TCP 3-way handshake, 3 segments will be sent (SYN, SYN/ACK, ACK). What happens if the third segment(ACK) is lost?

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In general , if client send syn, the server will return an ack, and server will set a expired time named "RTO", which means "retranmission time out";if client ack didnt arrive in time , it will trigger "tcp retranmission " ; if client ack didnt arrive in time again, send "tcp retranmission " again and set "RTO" to double or incrase in power; In the end , the server will think client has lost , so server will send "Reset " to client and tell client to release this connection; Most devices use this way to connect and release, but if there is a firewall , then the case is different

That depends on the protocol. If it is a protocol where the client sends data first, then after the ACK, there will be a data packet which contains the same ACK value, so all is fine.

If it is a protocol where the server sends data first, it will retransmit the SYN/ACK until it does receive an ACK and then all is fine.