What's the difference between your channel vs. concatenating multiple subreddits in the URL like /r/programming+science? Because the latter supports more than 3 subreddits at once.
No difference at all. I actually am concatenating the subreddits for the URL. It's on my to-do list to allow any number of subreddit per channel. I just haven't figured out an elegant way to do it on the mobile, yet.