When we perform traceroute it always start with ttl value 1 and send 3 packets. what information present in those 3 packets, and why we always get port unreachable message can we change the message?

Here we go

Lets say i want to trace google server, first of all pc will send 3 packets(it is probe count- you can change it) with TTL 1 , in very first hop it will expire and device will send icmp ttl exceed message now this process will continue untill it reaches its destination.Once it reaches its destination, device will send port unreachable message using icmp  just to let the source know it reaches to the destination.We can not change this thing.