Kotaku cites sources that can confirm the three GTA games are in the final stages of development. All three will be remastered using Unreal Engine, and will be a mix of “new and old graphics”. One of the sources say that the result of that mix looks like a heavily-modded game in the series. There are some minor updates as well, like with the UI, but overall all three remasters will try to stay true to their PS2 era originals.
The report also states that all three remastered GTA titles will be available on all platforms. That would mean previous and current generation consoles, PC, and the Nintendo Switch. As for their release date, the new Kotaku report says the current plan is for all three GTA remasters to be released within November of this year. At least, that would be the case for the console releases, since they are given priority. The PC ports may slip into next year, alongside possible mobile versions as well. But for any of the applicable platforms, all three remasters will be released as a single package. (Source: Kotaku. Images: Steam [1], [2])