I have 3 instances running on amazon ec2
instance 1 should be called myname.com, instance 2 and 3 should be called node1.myname.com and node2.myname.com respectively. How can this be achieved ? (can this be achieved ?)
Well, Route 53 is a option but is nothing like a easy solution. Amazon DNS is painful. You will need to write a script to dynamically update the dns to match what you want.
There is a whole bunch of other DNS services like zerigo ou dyndns that accept either REST calls to change the DNS or have a tool to automatically update the DNS.
Painful, but the only way. Another solution is to use Elastic IP's, but they are limited to 5 per region.
If you have a Elastic IP you can even request to have amazon resolve their reverse DNS for that IP. It is especially useful when you need to have a mail server.