# com.foo.www
RewriteCond %{HTTP_HOST} ^www\.foo\.com$
RewriteCond %{REQUEST_URI} !^/www/
RewriteRule (.*) /www/$1

# com.foo.drupal
RewriteCond %{HTTP_HOST} ^drupal\.foo\.com$
RewriteCond %{REQUEST_URI} !^/drupal/
RewriteRule (.*) /drupal/$1

#com.bar.beta
#RewriteCond %{HTTP_HOST} ^beta\.bar\.com$
#RewriteRule (.*) /drupal/$1