I have three domain names and one virtual dedicated hosting account. How can I host all three domains there?

Question

example domain1 should point to /var/www/html/1/
domain2 should point to /var/www/html/2/

Answer 1

Check out apache with virtual host.

Answer 2

Does your hosting account include a Control Panel of some sort? Many do, if not you can add one, it will make setting up additional sites very easy.

Now you say you have a "virtual dedicated" account, usually a dedicated account means you have your own server and nobody else is using it. With those you have full access to the apache.conf files so entering the new domains into that file under "Virtual Domains" is all that is necessary.
If on the other hand your site is a Virtual domain you probably do not have access to those configs. Its hard to answer unless we know how your server is configured.

Some hosts have multiple domain hosting even for "Virtual Domains" meaning you should be able to set these up. (Most of these will have a Control panel to do this!)

If you have a true dedicated server just install Webmin or DSM, Cpanel, ISPConfig or similar software and make it easy on yourself.
http://ispconfig.org/
http://webmin.com/

Good luck.