How do you make an object that always follows another object?

Right now, I'm trying to make sure that a game object follows a game object and switches to another when a button is pressed. But for now, this seems to change the position when the button is pressed, but it does not follow the object.

public Transform PosObjOne;

Public transformation PosObjTwo;

Public transformation PosObjThree;

void Update () {



if (Input.GetButtonDown ("UseVr"))
{
PosObjThree.position = PosObjTwo.position;
}

if (Input.GetButtonDown ("NoVr"))
{
PosObjThree.position = (PosObjOne.position);
}
}