I can’t formulate a rule for a redirect - how to do it right?

Question

htaccess

Good afternoon!
Task: redirect all files of the form
site.ru/cat1/cat2/file.html
to:
site.ru/cat1/cat2/cat3/file.html
Line
RewriteRule ^cat1\cat2(.*)$ /cat1/cat2/cat3/ $1 [R=301] does not pick up

Answer 1

RewriteRule ^(cat1/cat2)/([^/]+\.html)$ /$1/cat3/$2 [R=301,L]