Vector3 does not contain a constructor that takes 3 arguments

I'm not sure why this is happening. I'm a beginner to scripting, but I was under the impression that Vector3 took an X, Y and Z - why won't it take 3 arguments? in this case?

 using UnityEngine;
 using System.Collections;
 
 public class Vector3 : MonoBehaviour {
     
     Vector3 position = new Vector3 (0.0f, 3.0f, 4.0f);
 
 
     // Use this for initialization
     void Start () {
         print (position.x);
         print (position.y);
         print (position.z);
         print (position.magnitude);
         
         position.Normalize ();
         print ("Now with normalization");
         print (position.x);
         print (position.y);
         print (position.z);
         print (position.magnitude); 
 
 
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }