during installation, you usually need to change the chmod of config.php to 666 then back to 644 aftwards.
also, in most cases, if the folders mentioned above are not set to 777 phpbb3 will not let you complete the install.
I have completed numerous phpBB installations (and I'm talking about manual installations, not through Fantastico) and not even once have I had to change config.php perms to 666. Of course, this depends on the server setup, but on SiteGround servers phpBB installations work out of the box, you simply extract the package and proceed with the installation.
I see you are tech support for siteground.
you should do a search here for siteground and you will see many, many problems that have been caused by the way that siteground sets up their servers etc
The SiteGround support team, me included, is ready to help anyone who is experiencing issues with the installation of any application, not just phpBB. And if you do a search here for siteground and read some of the posts, you'll see that the reported problems are rarely caused by our server setup.
Anyway, I don't want to turn this thread off topic, and I'm sure you had good intentions when you said this, so I'll just finish off by saying that any forum member is welcome to PM me with any questions/problems regarding our servers