htaccess Elite

I'm having a hard time understanding .htaccess' mod_rewrite. Could someone help me with this problem...

I have a document root in /home/user/public_html/
I have 3 domains pointing there: http://www.domain[1|2|3].com

I want to transfer all of these domains to /home/user/public_html_2/

I cannot modify virtual hosts in apache's configs so i can only do it with .htaccess. So if i had a file /home/user/public_html/files/1.gif that was accessible by http://www.domain[1|2|3].com/files/1.gif - now i want this file to be located in /home/user/public_html_2/files/1.gif and accessed with exactly the same url.

Thanks for your help