Baldur's Gate 3 has three acts. Additionally, there is a short prologue before Act 1 and an epilogue after Act 3. The game structure is generally divided as follows:

• Prologue: A short introductory section.
• Act 1: Covers areas like Druids Grove, Goblin Camp, and the Underdark.
• Act 2: Set in the Shadow-Cursed Lands with major confrontations.
• Act 3: Takes place in Baldur’s Gate itself and serves as the final chapter of the story.

So, technically, it can be considered four parts including the prologue, but the main game is structured in three acts.