using UnityEngine.UI;

 if (health_value == 3) {
             GameObject.Find("health").GetComponent<Image>().color.a = 1;
             GameObject.Find("health1").GetComponent<Image>().color.a = 1;
             GameObject.Find("health2").GetComponent<Image>().color.a = 1;

         }

error CS1612: Cannot modify a value type return value of `UnityEngine.UI.Graphic.color'. Consider storing the value in a temporary variable