Url rewrite in htaccess

Hi experts,

I need some help with the Rewrite rules in drupal htaccess.

I've installed a drupal multisite, with three databases for three different languages.
Now I've put the database settings in directorys like:
/de.my-domain.com
/fr.my-domain.com
/en.my-domain.com

The problem is, my site can be accessed with several domains like:
my-first-domain.com
my-second-domain.com
...

I think, I have to set the Rewrite engine ON in htaccess file, and redirect all incoming requests to one subdomain per language so that:
fr.my-first-domain.com or fr.my-second-domain.com are redirected to fr.my-domain.com.

Frankly, regular expressions don't seem very regular to me and I have real problems to decrypt them ... even more as it is friday evening and some friends are calling me every 10 minutes from our favourite pub ...

Could anybody please help me to solve this problem, even if I didn't make a real attempt to solve it for myself (only 15 minutes trying ...)

Greetings from Austria
anschinsan