How can I make a 3rd person camera?

Well, first you'll want to go into a LocalScript in StarterGui and start out with this:

wait(.01)

while wait() do
game.Workspace.CurrentCamera.CameraType="Scriptable"
end

This will set the camera off of the character's Humanoid and allow you to code about. For this method, I suggest playing around with CoordinateFrame. I also suggest, to make it easier, setting to a part's CFrame. Such as character's Torso or Head.

The second method is simply resetting the CameraSubject. By default it's the character's humanoid.

Here's the way to do that:

wait(.01)

while wait() do
game.Workspace.CurrentCamera.CameraSubject=part --Set this to the part to set the camera to.
end

If I helped, hit that accept!

Also, if I missed something, just comment. :D