I saw the original TRON during its theatrical run, and I really enjoyed the tie-in with Legacy and Uprising.
Tron: Legacy made 400$ million worldwide in box office. Not sure what it made on the DVD / Blu-ray front, but just looking at the total Box Office:
Film studios, on average, take in about 50% of the gross. Different in different countries, territories, but for the sake of simplicity.... Disney got half of the $400 million, or $200 million. Put that against the movie's budget well over $100 million, plus marketing and distribution costs, and Disney MAYBE broke even. They could have even LOST money on the film.
And then Tomorrowland tanked (and to me, seemed to be only about 75% of the way "there"), and Disney flipped quickly out of the futuristic / sci-fi premise on its own intellectual properties, and switched to the newly acquired Marvel and Lucasfilm IPs. And THEY have made the Mouse very, very rich...
I'm afraid the window was missed for TRON 3. You're not going to get the actors back 5 or 10 years from now to do this.