Even though it didn't necessarily set the box office on fire, there's never been much doubt that Disney would develop a sequel to Tron: Legacy... Eventually. People started talking about a third Tron film almost immediately after the second was finished, but that was back in 2010.
It's possible this is just a working title, but Production Weekly previously let the world know that Terminator 5 would be called Terminator Genisys (a name too dumb to make up).
In any case, Tron 3 will reportedly start filming in Vancouver in October.
I know Legacy was met with mixed reactions, but I personally enjoyed it. So I say bring on more Tron.