traffic redirection by domain name to external addresses

For port redirection:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 8080

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 8443

From one IP to another and one port to another:

iptables -t nat -I PREROUTING --src 1.1.1.1 --dst 2.2.2.2 -p tcp --dport 80 -j REDIRECT --to-ports 8080

For the redirection of requests from host1 to host2 or host3 (assuming you are loadbalancing), I would probably actually use ldirector and not iptables.

If you are also controlling your own DNS, you might use round-robin DNS to do this.