Use the sample "How to Rotate and Move a Camera" from H:\3DGraphics\XNA Samples as a starting point for this exercise. Refer to these camera notes for assistance.
Add a forth camera mode. Place a fixed camera somewhere in the world. Arrange it so that the camera is always pointing at the avatar.
In the "over the shoulder camera mode" some lag to the forward and back movement so that the camera does not respond instantly to the movement of the avatar.
In the "over the shoulder camera mode" add functionality to allow the user to rotate the camera around the avatar in order to view the avatar from the side or the front.