If you use the FTP client FileZilla, it will tell you what the proper FTP path is. (I don't know about other clients because FileZilla is what I use). In FileZilla the path will appear in the "Remote Site" bar. Most FTP clients tell you the path to the directory you have opened.
Otherwise, you can "build" it manually. Sign into FTP with your FTP client, and for every directory you enter, note it.
So I start at the root
/
Enter the "www" directory
/www/
Enter the "example.com" directory
/www/example.com/
And finally "phpbb3"
/domains/example.com/phpbb3/
That would be my proper FTP path.