HitFilm's compositor can work in 2D and 3D. Depending on the nature of your project you may use one or both of these modes.
3D should not be thought of as 'better'. Although it does provide more options for compositing, it is also more complex than 2D layer compositing. For many shots 2D compositing will be a faster and easier method.
2D composite shots can only have 2D layers.
3D composite shots can include both 2D and 3D layers.
Changing to 3D
To change a composite shot into 3D you need to add a camera. This can be done using the New Layer menu at the top of the composite shot timeline.
A default camera will be added and the scene will automatically switch into 3D.
For more information on cameras see Camera layers.
Initially the 3D view looks very similar to the 2D view, at least until you move the camera. See Controlling the 3D view for more information.
If you add a 3D effect or switch a 2D layer to 3D, a camera will be automatically added to the composite shot.
Reverting to 2D
If you want to change back to a 2D composite shot, you need to remove all your camera layers from the timeline.
Any 3D layers in your composite shot will be disabled if you have no active cameras.