Timer Tick not increasing values on time interval

You can use ViewState to store and then read the value of i again.

int i = 0;

protected void Timer1_Tick(object sender, EventArgs e)
{
    //check if the viewstate with the value exists
    if (ViewState["timerValue"] != null)
    {
        //cast the viewstate back to an int
        i = (int)ViewState["timerValue"];
    }

    i++;

    Label3.Text = i.ToString();

    //store the value in the viewstate
    ViewState["timerValue"] = i;
}