What is difference between inside point, centre of gravity and centre point transformer ?

These used to be different transformers, but are now all integrated into the CenterPointReplacer transformer, with a different mode for each type.

The functionality is described in the documentation, be sure to expand the "Mode" parameter description:

http://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Transformers/Transformers/centerpointreplacer.htm

• Center Point of Bounding Box: Replaces the geometry of the feature with a point that is in the center of the feature's bounding box. Depending on the feature’s shape, the resulting point may be far outside of the original feature.
• Center of Gravity Point: Replaces the geometry of the feature with a point at the center of mass of the feature. Depending on the feature’s shape, the resulting point may be far outside of the original feature.
• Any Inside Point: Replaces the geometry of the area feature with a point that is guaranteed to be inside the area.