public class randommessage : MonoBehaviour {
    int message = 0;   

    public void Start()
    {
       message = Random.Range(1,3);
    }

    public void OnGUI()
    {
       if(message == 1)
         GUI.Label(new Rect(Screen.width/2, Screen.height/2, 500, 30), "Text1");
       if(message == 2)
         GUI.Label(new Rect(Screen.width/2, Screen.height/2, 500, 30), "Text2");
       if(message == 3)
         GUI.Label(new Rect(Screen.width/2, Screen.height/2, 500, 30), "Text3");
    }
}

Try something like this:

string[] texts = new string[]{"Text1", "Text2", "TEXT THREE"};

Then, when you want to get a random one, use this line:

string currentText = texts[Random.Range(0, texts.Length)];