Every single game in the entire Grand Theft Auto series is Three-Dimensional (3D). This means that 3D models are used, and the camera/view moves around them; as opposed to Two Dimensional (2D) which involves only animated pictures.
Many people believe that GTA 1, GTA 2 and GTA Advance are 2D, but this is not the case. The top-down perspective looks to be 2D, and many aspects of the game are (including the vehicles and pedestrians). However the player can clearly see the sides of the buildings when going around them, which are drawn on-the-fly from 3D models.