https://sitename.ru/catalog1/catalog2/page-name[1,2,3...].html
https://sitename.ru/catalog1/page-name[1,2,3...].html
RedirectMatch 301 ^/(catalog1)/[^/]+/([^/]+\.html)$ /$1/$2
RewriteEngine on
RewriteRule ^(catalog1)/[^/]+/([^/]+\.html)$ /$1/$2 [R=301,L]