static class Program
{

        public static MyTest1 MyObject = new MyTest1();
...
}

    public class MyTest1
    {
        public MyTest2 Test2 { get; set; } = null;
        public string s = "Foo"; 
    }

    public class MyTest2
    {
        public int MyInt = 123;
        public string MyString = "My String 123";       
    }

    public class MyTest3
    {
        public MyTest2 Test2 = Program.MyObject.Test2;     
    }

    public partial class Form1 : Form
    {

        public MyTest3 test3 = null;

        public Form1()
        {
            InitializeComponent();
        }

        public void Print(string s)
        {
            textBox1.AppendText(s + "\n");
        }

        private void button1_Click(object sender, EventArgs e)
        {
            test3 = new MyTest3();
            Print("test3 initialized");
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Program.MyObject.Test2 = new MyTest2();
            Print("Program.MyObject.Test2 initialized");
        }

        private void button3_Click(object sender, EventArgs e)
        {
            if (test3.Test2 == null) Print( "... but why ?" );
        }
    }

public class MyTest3
    {
        public MyTest2 Test2 { get => Program.MyObject.Test2; }    
    }