Two IF Statements not working in one method

I'm making my own first person controller and I want to have 3 movement speeds: walk, run and sprint. I've got my character to walk and sprint, but not running. When i flip the two if statements in the ChangeMovementSpeed() method, this time walking and running works but not sprinting. I'm not exactly new to C# (around 6 months) but it must be my stupidity and lack of experience that's causing this problem. Any help is appreciated. Thanks.

My First Person Controller code:

Note: I'm using Naughty Attributes for organization purposes :)

 using System;
 using UnityEngine;
 using NaughtyAttributes;
 
 namespace ZGS
 {
     [RequireComponent(typeof(CharacterController))]
     public class ZFirstPersonController : MonoBehaviour
     {
         #region Variables
             #region Settings
                 #region Public
                     [BoxGroup("Settings")] public float walkSpeed;
                     [BoxGroup("Settings")] public float runSpeed;
                     [BoxGroup("Settings")] public float sprintSpeed;
                     [BoxGroup("Settings")] public float jumpHeight;
                     [BoxGroup("Settings")] public float gravity = -9.81f;
                     [BoxGroup("Settings")] public CharacterController characterController;
                 #endregion
                 #region Private
                     [BoxGroup("Settings")] private float movementSpeed;
                     [BoxGroup("Settings")] Vector3 velocity;
                 #endregion
             #endregion
             #region Ground Check
                 #region Public
                     
                     [BoxGroup("Ground Check")] public float groundDistance;
                     [BoxGroup("Ground Check")] public LayerMask groundMask;
                     [BoxGroup("Ground Check")] public Transform player;
                 #endregion
                 #region Private
                     [BoxGroup("Ground Check")] bool isGrounded;
                 #endregion
             #endregion
         #endregion
 
         #region Standard Methods
         void Update()
         {
             ExecuteGroundCheck();
             ChangeMovementSpeed();
             ExecuteMovement();
             ExecuteJump();
             ExecuteGravity();
         }
         #endregion
 
         #region Custom Methods
         void ExecuteGroundCheck()
         {
             isGrounded = Physics.CheckSphere(player.position, groundDistance, groundMask);
 
             if (isGrounded && velocity.y < 0)
             {
                 velocity.y = -2f;
             }
         }
 
         void ChangeMovementSpeed()
         {
             if (Input.GetKey(KeyCode.LeftShift))
             {
                 movementSpeed = runSpeed;
             }
             else movementSpeed = walkSpeed;
 
             if (Input.GetKey(KeyCode.LeftAlt))
             {
                 movementSpeed = sprintSpeed;
             }
             else movementSpeed = walkSpeed;
         }
 
         void ExecuteMovement()
         {
             float x = Input.GetAxis("Move");
             float z = Input.GetAxis("Strafe");
 
             Vector3 move = transform.right * x + transform.forward * z;
             characterController.Move(move * movementSpeed * Time.deltaTime);
         }
 
         void ExecuteJump()
         {
             if (Input.GetButtonDown("Jump") && isGrounded)
             {
                 velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
             }
         }
 
         void ExecuteGravity()
         {
             velocity.y += gravity * Time.deltaTime;
             characterController.Move(velocity * Time.deltaTime);
         }
         #endregion
     }
 }