Apache URL Rewriting - Hide path in URL only when accessing specific path - apache

Question

I would like to rewrite my URL only when typing this:
www.domain.com/dir1/dir2/dir3/
in that path there is a index.html file that will be automatically loaded. And I'd like it to become this:
www.domain.com/index.html
It is important that the root www.domain.com does not redirect to that subdirs because it already redirects to another .php file.
I wrote the following .htaccess file:
RewriteRule ^dir1/dir2/dir3/(.*)$ dir1/dir2/dir3/$1 [L]
but it redirects even when typing the root domain.com.
Could you help me with that?
Edit:
I have tried in another browser and the real problem is that when typing domain.com/dir1/dir2/dir3 or domain.com/dir1/dir2/dir3/index.html the dir1/dir2/dir3/ is still shown and is not hidden.
Edit 2:
my complete .htaccess:
AddHandler application/x-httpd-php56s .php
ErrorDocument 404 /404/404.html
ErrorDocument 500 /404/404.html
RewriteEngine on
RewriteBase /
Options -Indexes
RewriteCond %{REQUEST_URI} ^/404/$
RewriteRule ^(.*)$ /404/404.html [R=404,NC,L]
RewriteCond %{REQUEST_URI} ^/500/$
RewriteRule ^(.*)$ /404/404.html [R=500,NC,L]
Thanks

Answer 1

Replace your current code with this:
DirectorySlash On
RewriteEngine On
# skip all known resources from rewrite rules below
RewriteRule \.(?:jpe?g|gif|bmp|png|ico|tiff|css|js)$ - [L,NC]
# external redirect from actual URL to pretty one
RewriteCond %{THE_REQUEST} \s/+dir1/dir2/dir3/(\S*) [NC]
RewriteRule ^ /%1 [R=301,L,NE]
# internal forward from pretty URL to actual one
RewriteRule ^(?!dir1/dir2/dir3/)(.*)$ dir1/dir2/dir3/$1 [L,NC]