Well, this is my take on what's going on. I don't have any kind of inside track, sources, or anything like that. It's just educated guesses.
I think Tron 3 is still going to happen, but it's probably stalled at the moment. Alan Horn, who's in charge of the Disney film unit now (he was a head honcho at Warner Bros., before), was bullish on another Tron sequel and I don't think that's changed. He probably is still pushing for it to be made. Eventually. I believe Disney wants to take one last crack at turning Tron into a major franchise.
I'm afraid what's happened, is what some of us feared and predicted earlier. Myself included. Namely, Star Wars.
Episode VII is the number one priority at Disney films, right now. Almost guaranteed to be a big box office hit. As a result, probably a lot of other things have been pushed aside. That includes Tron. Tron is still a risk, Star Wars isn't.
I think the recent news stories about Tron 3 - which are really just regurgitating old information - are a measure to help keep up interest in another Tron sequel, to show the powers that be that there's still something of a demand for it.