Keep gravity while adding velocity to a Rigidbody?

Little help for a newbie. So I have a player "sphere" which moves around on the x using addforce with the a and d keys. I tried to implement a jumping feature, but I have this issue where the ball, if force is added on the x on during the fall, will seem to be unaffected by gravity and move only to the side where velocity is added. Here is the code:

 using UnityEngine;
 using System.Collections;
 
 public class movementx : MonoBehaviour {
 
     public int speedleft;
     public int speedright;
     public int jump;
     public int downjump;
      public Rigidbody ball;
 
     void Update () {
     
 
 
     if (Input.GetKey (KeyCode.D)) {
             ball.velocity = Vector3.zero;
             ball.angularVelocity = Vector3.zero;
             ball.AddForce (speedleft, 0, 0);
         } else if (Input.GetKey (KeyCode.A)) {
             ball.velocity = Vector3.zero;
             ball.angularVelocity = Vector3.zero;
             ball.AddForce (speedright, 0, 0);
 
         } else if (Input.GetKeyDown (KeyCode.Space)) {
             ball.velocity = Vector3.zero;
             ball.angularVelocity = Vector3.zero;
             ball.AddForce (0, jump, 0);
         }
     }
 }

Thanks again for the help!