@carlossketches my wife and I met on Wet Canvas when they only had about 30,000 members in 2003. We met as moderators in the private forums there, she was the lead for sculpture and textiles, I was the lead for drawing and watercolors. The community is now OVER 100,000 strong, and divided by genre and medium. It's a great place for beginners but the involvement depends on the " project " going on at the time. I was the creator of the Friday Night Drawing Event, which has morphed into the Weekly Drawing Event, and run by individual members I suppose as we had by volunteering.
Basically the event host puts up a bunch of images under a theme, and you have I believe an allotted time to choose one, use any medium you prefer and post it for discussion, while you are asked to post on other artists work... we also had a chat associated with it when I ran it, which is where I met my wife... 12 years married this coming Saturday!
Deviant Art's Forum has sections for projects, genre and styles, as well as a monthly contest. Of course there is always a Xmas themed one for prizes held by various groups, there's also a Holiday Card Project where all those that enter get a free month of premium content.
Art communities as a whole online are based on the medium you want to specialize in, or a genre you enjoy... for those who like to see highly professional work, there is Behance, not really a community, but you can see WIPS in dozens of styles, programs, globally...
I was gonna mention ConceptArt.org... but the site is ALWAYS down, and from reviews I've read online tonight, the quality and community has been going down hill since 2010. Suffice to say this place used to offer daily critiques in a variety of mediums and styles. Unfortunately it isn't working half the time you try to log in.
Anyway, you said 3, so there ya go... just kidding, here have a look at THIS...