How to setup three domains to use one DNS and mail server

[Don Smith]

Hi, we have domain.net,
and have just purchased domain.com and domain.org.
Our users have their email use domain.net.
Now that we have domain.com and domain.org, how can I set up my DNS server
to recognize these new domains so it points to us?
Also, so our users can get email either from domain.net or domain.com?
Thanks very much!
Don