You can use:
ErrorDocument 404 /404.html
RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain\.org$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301,NE]
RewriteCond %{REQUEST_FILENAME} \.(gif|jpe?g|png|js|css|swf|php|ico|txt|pdf|xml)$ [OR]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -l
RewriteRule ^ - [L]
RewriteRule ^([^/]+)/([^/]+)/([^/]+)/?$ index.html?x=$1&y=$2&z=$3 [L,QSA]
RewriteRule ^([^/]+)/([^/]+)/?$ index.html?x=$1&y=$2 [L,QSA]