Your explanation is a little hard to follow.
iptables -t nat -A POSTROUTING -p tcp --source 192.168.50.159/32 -j SNAT --to-source 192.168.50.159:3057
This will take the source IP address and port of 192.168.50.159:any-tcp and change it to 192.168.50.159:3057-tcp for any packet that leaves the machine (as long as it is an exact match of the specified IP address). You would enter this rule on the box that has an IP of 192.168.50.159. You could make another rule for udp.