Open Left 4 Dead 2
Go to Keyboard Mouse Settings, Allow Developer Console.
Play a Random Single Player Campaign.
Open Console Commands with this key -> " it's the key on the left of the number 1.
Type this text, copy one verse and just put one by one, that should work without problem.

Bind alt thirdpersonshoulder
c_thirdpersonshoulderoffset 0
c_thirdpersonshoulderaimdist 720
c_thirdpersonshoulderdist 50
c_thirdpersonshoulderheight 0
c_thirdpersonshoulderoffset 20

Easy and simple tutorial about the commands.

Bind Alt thirdpersonshoulder - Alt is a key who most people don't use, so is very easy to alternate from first person to third person.
c_thirdpersonshoulderaimdist - setting for the crosshair aim in third person, 720 value is the default and recommended one.
c_thirdpersonshoulderdist - setting for close or far camera in third person, putting a large value makes the camera so far away.
c_thirdpersonshoulderheight - height of the camera in third person, 0 value is the default one, but you can change for you like.
c_thirdpersonshoulderoffset - setting for the camera on the character shoulder or centered in the screen.