Input.mousePosition is a Vector3 already....

Are you working in C# or javascript? You tagged the question with c#

in C# (C# scripts must be named the same as their class name, in this example CamMouse)

using UnityEngine;
using System.Collections;

public class CamMouse : MonoBehaviour {


    void Update () {
        transform.position = Input.mousePosition;
    }
}

in Javascript

function Update () {
	transform.position = Input.mousePosition;
}

or, for fun and syntax similar to yours, in C#

using UnityEngine;
using System.Collections;

public class CamMouse : MonoBehaviour {


    void Update () {
        transform.position = new Vector3(Input.mousePosition.x,
             Input.mousePosition.y,
             Input.mousePosition.z);
    }
}

Also, note that Input.mousePosition goes between (0,0,0) in the lower left corner, to (Screen.Width, Screen.Height, 0) in the upper right.