I did step by step the points from here
I upload 3.3.3, run the install.Decompress the phpBB3 archive to a local directory on your system.
Upload all the files contained in this archive (retaining the directory structure) to a web accessible directory on your server or hosting account.
Change the permissions on config.php to be writable by all (666 or -rw-rw-rw- within your FTP Client)
Change the permissions on the following directories to be writable by all (777 or -rwxrwxrwx within your FTP Client):
store/, cache/, files/ and images/avatars/upload/.
Point your web browser to the location where you uploaded the phpBB3 files with the addition of install/app.php or simply install/, e.g. http://www.example.com/phpBB3/install/app.php, http://www.example.com/forum/install/.
Click the INSTALL tab, follow the steps and fill out all the requested information.
Change the permissions on config.php to be writable only by yourself (644 or -rw-r--r-- within your FTP Client)
phpBB3 should now be available, please MAKE SURE you read at least Section 6 below for important, security related post-installation instructions, and also take note of Section 7 regarding anti-spam measures.
If you experienced problems or do not know how to proceed with any of the steps above please read the rest of this document.
I connected with the DB where I imported the back up I've already had.
the result is the same thing. one post, one user.