How do i freeze the player for the firs 3 second?

Hello guys, i’m new with this program and with the C#, i would like to know how can i freez the player for the first 3 second of the game?

This is the script that i used:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

public class PlayerMotion : MonoBehaviour {

private CharacterController controller;
private Vector3 moveVector;

private float speed = 6f;
private float verticalVelocity = 0f;
private float gravity = 12f;

private float animationDuration = 3.0f;
public float startTime;

public bool isDead = false;
//

//
void Start () {

	controller = GetComponent <CharacterController> ();
	startTime = Time.deltaTime;
				}


void Update () {
	
	if (isDead) {
		return;
	}

	if (Time.time - startTime < animationDuration)
	{
		controller.Move (Vector3.forward * speed * Time.deltaTime);
		return;
	}

	moveVector = Vector3.zero;

	if (controller.isGrounded) 
	{
		verticalVelocity = -0.5f;
	} 
	else 
	{
		verticalVelocity -= gravity * Time.deltaTime;
	}

	//X 
	moveVector.x = Input.GetAxisRaw ("Horizontal") * speed;
	if (Input.GetMouseButton (0)) 
	{
		if (Input.mousePosition.x > Screen.width / 2)
			moveVector.x = speed;
		else
			moveVector.x = -speed;
	}
	//Y
	moveVector.y = verticalVelocity;
	//Z
	moveVector.z = speed;

	controller.Move (moveVector * Time.deltaTime); 
}
public void SetSpeed(float modifier)
{
	speed = 6f + modifier;
}

private void OnControllerColliderHit (ControllerColliderHit hit)
{
	if (hit.collider.tag == "Obstacol")
		Death();
	}

public void Death ()
{
	isDead = true;
	GetComponent <Score> ().OnDeath ();
		
}

}

Thank you guys

You can do something pretty simple like this:

private float tActive;
public float tWait = 3.0f;  // set this in inspector to any other value in seconds, if desired

void Start() {
    tActive = 0.0f;
}

void Update() {
    tActive += Time.deltaTime;
    if (tActive < tWait) {
        // do whatever your waiting action is
        return;
    }

    // do your action here
}