Phpbb3 will tell you when you post a reply if a new post has been added to the topic since you started editing your reply. This way you can decide if you need to update your post to reflect the most recent post in the topic.
i’m not sure about any existing plug-ins, but i think that by default topics update real time while you are typing a reply. also if you have the presence enabled setting enabled, you can see who is currently typing a reply at the same time you are.