The Events of Disc 1 of the original PS1 Game
The Events of Disc 2 of the original PS1 Game
The Events of Disc 3 of the original PS1 Game
So, Final Fantasy VII Remake Part 3: Ground Zeroes?
Honestly though, if done in three parts, the best case scenarios would be this:
Part 1 - The events of Midgar, which is pretty much Act I of FFVII.
Part 2 - Act II which covers Kalm with the flashback of Sephiroth and will end when Cloud hands over the Black Materia to set the main conflict.
Part 3 - The start of Act III in Junon all the way to the end of the game.