3D mouse position

Hi, I'm trying to make a script for a free throw ball game and need to get the mouse position in the world, and it should be in 3D to be able to throw in all 3 axes. I'm kinda new to scripting and the script I wrote works, but not right. I am not sure how to get the depth or the y axis working, because the screen is only 2d.

 using UnityEngine;
 using System.Collections;
 
 public class ShootBall : MonoBehaviour {
 
     private Rigidbody rb;
     private RaycastHit hit;
     private Vector3 com;
     private Vector3 shootDirection;
 
     void Start () {
         rb = GetComponent<Rigidbody>();
     }
 
     void Update () {
     }
 
     void OnMouseDown (){
         com = rb.worldCenterOfMass;
         Debug.Log (com);
     }
 
     void OnMouseDrag (){
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         Debug.Log(ray);
         Physics.Raycast(ray, out hit);
     }
 
     void OnMouseUp (){
         shootDirection = com - hit.point;
         rb.AddForce (shootDirection * 100);
     }
 }