Thats sounds nice and relativ easy.
The Main Problem is in my opinion not a third person view, it is a intelligent Camera Movement. In a FPS its bad when your cam move in a wall.
I created a "dummyboy" a invisible unit fixated to my Real Playerunit. The Follow cam is fixated on the dummy and with waypoint length measurments i can check cliffs and other objects. When a object is in the way the dummy have a new position in relation to the Real Player.