2.5d Player Controller

Hello, I am making a 2.5 d game , like with camera view on 2d and with 3d models (just explaining for some reasons ,i know you guys know about 2.5d) , Just have a problem about controlling ,i don't want to my character to go to x axis (in 2d i know it's x and y) in 2.5 d i have to lock z axis , but my player is 3d and it is looking at z so i need my player to go forward with it's look and i want to lock x axis so my player smoothly move to y(up/down) and z(forward/backward),I tried that -

 using UnityEngine;
 using System.Collections;
  
 public class force : MonoBehaviour {
  
     // Use this for initialization
     void Start () {
  
     }
    
     // Update is called once per frame
     void Update () {
         Time.timeScale = 0.5f;
         if(Input.GetKey(KeyCode.UpArrow))
            {
             //GetComponent<Rigidbody>().velocity = new Vector2(0, 12);
             GetComponent<Rigidbody>().AddForce(Vector3.up * 200,ForceMode.Acceleration);
         }
         if(Input.GetKey(KeyCode.DownArrow))
         {
             //GetComponent<Rigidbody>().velocity = new Vector2(0,-12);
             GetComponent<Rigidbody>().AddForce(Vector3.down * 200,ForceMode.Acceleration);
         }
         if(Input.GetKey(KeyCode.LeftArrow))
         {
             //GetComponent<Rigidbody>().velocity = new Vector2(-12, 0);
             GetComponent<Rigidbody>().AddForce(Vector3.back * 200,ForceMode.Acceleration);
         }
         if(Input.GetKey(KeyCode.RightArrow))
         {
             //GetComponent<Rigidbody>().velocity = new Vector2(12, 0);
             GetComponent<Rigidbody>().AddForce(Vector3.forward * 200,ForceMode.Acceleration);
         }
         }
 }
  

but it won't work at all , even if i tried to lock z with vector 2 but it still move to z but i want to lock x , and i tired vector 3 but it still won't work , it sill moving to x . any help, thanks .