reading date from popup window

Write the event for calender control.
for 3 textbox ,you have to create threee calender control and use the below event to get the selectedDate into the textBox.

protected void Calendar1_SelectionChanged(object sender, EventArgs e)
{
   
    TextBox1.Text = Calendar1.SelectedDate.ToString();
}

You can also use like this on your source page-

document.getElementById('textbox1').value = calendarControl.getDate();

Hope it helps.