public partial class Form2 : Form
{
    public string str;
    public Form2()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        str = textBox1.Text;
        Form3 f3 = new Form3();
        f3.ShowDialog(this);
    }
}

public partial class Form3 : Form
{
    public Form3()
    {
        InitializeComponent();
    }

    private void Form3_Load(object sender, EventArgs e)
    {
        Form2 f2 = (Form2)this.Owner;
        MessageBox.Show(f2.str);
    }
}