Back to main menu after 3s from show ui text

Easiest way to do it would be to attach a script to your text, and only activate your text's GameObject when you show it, so that you can call the code to load the scene with the OnEnable callback. To wait for 3s, you need to use a Coroutine.

 using UnityEngine;
 using UnityEngine.SceneManagement;
 using System.Collections;
 
 public class LoadScene : MonoBehaviour
 {
     [SerializeField]
     private int m_SceneIndex = 2;
 
     [SerializeField]
     private float m_DelayBeforeLoad = 3f;
 
     private void OnEnable()
     {
         StartCoroutine(WaitAndLoadScene(m_SceneIndex, m_DelayBeforeLoad));
     }
 
     private IEnumerator WaitAndLoadScene(int sceneIndex, float delay)
     {
         yield return new WaitForSeconds(delay);
         SceneManager.LoadScene(sceneIndex);
     }
 }

However, it would be better to write a public function that you would call right after showing the text, in which case you'd need to replace private void OnEnable() by public void Load().