There's even active HoMM3 PvP streamer community with several thousands live audience on Youtube and Twitch. There are also addons, and community, for Heroes 4 and 5, but two magnitudes smaller.
Glad to see it still alive and kicking! Who to watch?
https://www.twitch.tv/theknownworld - mostly weekly streams
https://www.youtube.com/user/TheKnownWorld - some informational videos (strategies and such) as well as challenges against computers. Great explanations on the why/how and you get to watch at your own pace. I enjoy these more than the streams. Unfortunately he has run out of challenge ideas I think hehe