phpBB installation procedure with SQLite3 has several issues:

1) Full path to the database should be entered, while the code gets it as a server name with optional port value, see https://github.com/phpbb/phpbb/blob/3.2.x/phpBB/phpbb/db/driver/sqlite3.php#L39.

2) There's a "Database name" field on the database configuration page during installation, but the value is never used in the code to connect with the database.

3) Connection uses SQLITE3_OPEN_CREATE flag, which means database to create if doesn't exist, but it will create 2 database files in /install and board root folders. This is pointless as /install folder is to be deleted/renamed as well as it's not safe like that. Also it's not documented anywhere. See https://github.com/phpbb/phpbb/blob/3.2.x/phpBB/phpbb/db/driver/sqlite3.php#L50