how to get two text box sum in third textbox without clickng on button ?

A couple of points:

- if you use the TextChanged event then you will be attempting to do the calculation with every keystroke in the textboxes - i.e. before the user has finished typing. I personally would choose an event that is fired once the user has completed their input e.g. the Validated event

- By using Convert.ToInt32 you are assuming that both textboxes are populated and definitely contain numbers. If txt_Item2 has not been populated then this will throw a FormatException - "Input String was not in a correct format". You are better off using int.TryParse[^] - see this CP article for more detail Difference Between Int32.Parse(), Convert.ToInt32(), and Int32.TryParse()[^]

This code works

private void textBox1_Validated(object sender, EventArgs e)
{
    CalcTotal();
}
private void textBox2_Validated(object sender, EventArgs e)
{
    CalcTotal();
}
private void CalcTotal()
{
    int i, j;
    if(int.TryParse(textBox1.Text, out i) && int.TryParse(textBox2.Text, out j))
        textBox3.Text = (i + j).ToString();
}

Or you can do it a little more efficiently like this

public Form1()
{
    InitializeComponent();
    this.textBox1.Validated += new System.EventHandler(this.CalcTotal);
    this.textBox2.Validated += new System.EventHandler(this.CalcTotal);

}

private void CalcTotal(object sender, EventArgs e)
{
    int i, j;
    if(int.TryParse(textBox1.Text, out i) && int.TryParse(textBox2.Text, out j))
        textBox3.Text = (i + j).ToString();
}