How to make randomizer randomize till it's the right number

I'm making a sudoku game and at first I'm doing small version 3x3. After some coding I couldn't find a way to:

How do I make S randomise till its the right number (S randomizes, Skaiciai.Neradau means it didn't find a repeating number, Skaiciai.Yra_Toks_Skaicius means he found the same number already used)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace _3x3
{
    public partial class Form1 : Form
{

    TextBox[] Boxai;

    public Form1()
    {
        InitializeComponent();
        Boxai = new TextBox[] { textBox1, textBox2, textBox3};
    }


    public enum Skaiciai
    {
        Radau_Toki_Skaiciu,
        Neradau
    };


    private void button1_Click(object sender, EventArgs e)
    {




        Random X = new Random();

        for (int i = 0; i < 3; i++)
        {
            int S = X.Next(1, 4);

            switch (i)
            {
                case 0:
                    int A, B;
                    if (textBox2.Text == "") { A = 0; } else { A = int.Parse(textBox2.Text); }
                    if (textBox3.Text == "") { B = 0; } else { B = int.Parse(textBox3.Text); }
                    Skaiciai R = PatikrinkArYraSkaicius(S, new int[] { A, B });
                    if (R == Skaiciai.Neradau)
                    {
                        textBox1.Text = S.ToString();
                    }
                    break;
                case 1:
                    int C, D;
                    if (textBox1.Text == "") { C = 0; } else { C = int.Parse(textBox1.Text); }
                    if (textBox3.Text == "") { D = 0; } else { D = int.Parse(textBox3.Text); }
                    Skaiciai T = PatikrinkArYraSkaicius(S, new int[] { C, D });
                    if (T == Skaiciai.Neradau)
                    {
                        textBox2.Text = S.ToString();
                    }
                    break;
                case 2:
                    int E, F;
                    if (textBox1.Text == "") { E = 0; } else { E = int.Parse(textBox1.Text); }
                    if (textBox2.Text == "") { F = 0; } else { F = int.Parse(textBox2.Text); }
                    Skaiciai Y = PatikrinkArYraSkaicius(S, new int[] { E, F });
                    if (Y == Skaiciai.Neradau)
                    {
                        textBox3.Text = S.ToString();
                    }
                    break;
            }

        }
    }


    Skaiciai PatikrinkArYraSkaicius(int _ieskomasSkaicius, int[] _skaiciai)
    {
        Skaiciai Rezultatas = Skaiciai.Neradau;
        for (int i = 0; i < _skaiciai.Length; i++)
        {
            if (_skaiciai[i] == _ieskomasSkaicius)
            {
                Rezultatas = Skaiciai.Radau_Toki_Skaiciu;
            }
        }
        return Rezultatas;
    }
}

}