I just added a phpBB3 mod which adds RSS feeds to a phpBB3 forum to the forums on this page. Like most phpBB3 mods, it’s a bit of a chore to install. More so since currently there is no easyMod for phpBB3, so everything has to be done manually. But if you follow the instructions in the install.xml to the letter, everything should be fine.
The only part needing some creativity will be your style. Unless you’re using prosilver, since that is the only style that is supported out of the box. Essentially, all that is really required is to add a link to the page <your forum location>/create_syndication.php. If you feel like being perfect, style that too. But as you may have noticed, my create_syndication.php is still of the plain vanilla variety.
The mod allows you to create feeds for seperate board or to combine several boards into a single feed. It also allows you to set whether you want to see all new posts in the feed, or just new topics / threads. Check it out and play around with it!