Only once in update

Yes. It's "Start" - called just before Update, called only when enabled, called only once.

Second option Is "OnEnable" - called always before Update, called every time Behaviour is enabled (so it's like Start but called every time Behaviour is re-Started).

Third option (not for newbies tho), if you really (and I mean reeeeeeeeally) need to use that Update calls (I'm pretty sure you do not) - would be to create field of type System.Action. It will allow you to group arbitrary number of method calls under one field and, interestingly enough, remove/add methods from that group - runtime.

 using UnityEngine;
 public class Example : MonoBehaviour
 {
     System.Action _onUpdate;
     bool something = true;
 
     void Awake ()
     {
         //initialize _onUpdate with one job:
         _onUpdate = new System.Action( JobA );

         //add JobB if something is true:
         if( something )
         {
             _onUpdate += JobB;
         }
     }
 
     void Update ()
     {
         //execute all methods in this group:
         _onUpdate();

         //word of warning: trying to execute a non-initialzed System.Action field
         //will throw NullException. You can fix this by initializing this field
         //(in Awake, for example) or testing for null just before execution
     }
 
     void JobA ()
     {
         Debug.Log("Tinky-Winky");
     }
 
     void JobB ()
     {
         Debug.LogWarning("Laa-Laa");

         //removes self when executed once:
         _onUpdate -= JobB;
     }
 
     void JobC ()
     {
         Debug.LogError("Po");
     }
 
     public void AddJobC ()
     {
         _onUpdate += JobC;
     }
 
     public void RemoveJobC ()
     {
         _onUpdate -= JobC;
     }
 
     public void AddVeryDifferentJob ( System.Action job )
     {
         _onUpdate += job;
     }
 
 }