RewriteCond %{HTTP_HOST} ^(?:www\.)?domain\.ru$ [NC]
RewriteRule ^(spb|msk|rostov)/(.*) http://$1.domain.ru/$2 [R=301,L]RewriteCond %{HTTP_HOST} ^(?:www\.)?domain\.ru$ [NC]
RewriteCond %{REQUEST_URI} ^/(msk)/(.*)$ [OR]
RewriteCond %{REQUEST_URI} ^/(spb)/(.*)$ [OR]
RewriteCond %{REQUEST_URI} ^/(rostov)/(.*)$
RewriteRule ^ http://%1.domain.ru/%2 [R=301,L]