https://www.youtube.com/watch?v=7x7zr1onxfU
Announced today on Twitter and Rockstar official site, but no more than vague terms are used:
https://www.rockstargames.com/newswire/article/3933o7ko43o839/grand-theft-auto-the-trilogy-the-definitive-edition-and-more-coming-so
...and the classic versions are going off the digital stores next week, as stated in the article.
https://support.rockstargames.com/articles/4408282103571/Grand-Theft-Auto-III-Grand-Theft-Auto-Vice-City-and-Grand-Theft-Auto-San-Andreas-not-available-for-purchase