Page titles in vanilla phpBB3 forums are ugly. To display them as {Forum/Page title} @ {Sitename} you need to edit viewforum.php and viewtopic.php (they are in the root of the forum) and overall_header.php in each style template:

viewforum.php & viewtopic.php

Find: page_header
Edit the line so it looks like: page_header($topic_data[’topic_title’]);

overall_header.php

Edit the content of the title tags so it looks like: {PAGE_TITLE} @ {SITENAME}

If you want to change the PAGE_TITLE of the index page, you can either edit page_header in index.php or the INDEX string in the language packs.