Hacker News Reader

Depending on the subreddit, reddit comments could be worth combining in as well.

As to "how" to accomplish it, my mind is immediately jumping towards a browser extension that automatically searches the other sites for matching content when you're viewing the discussion page on any of them, and then incorporates the relevant info somehow.

I made a small Chrome extension for that, it shows links to all discussions about the URL on HN and Reddit. Viewing comments on one page would be cool, but it is probably a copyright infringement.

You could do three searches? Perhaps that's not convenient enough.

That could be a start for a prototype browser plug-in.