C#

using (StreamWriter sw = new StreamWriter("File.txt")) 
{
  sw.WriteLine(textBox1.Text);
  sw.WriteLine(textBox2.Text);
  sw.WriteLine(textBox3.Text);
}

// Put in constructor of form
this.button1.Click += new EventHandler(this.button1_Click);
// Put inside form definition
void button1_Click(object sender, EventArgs e)
{
  // Here goes the code to save the file.
}

void textBoxXX_KeyPress(object sender, KeyPressEventArgs e)
{
  // Check whether pressed character is number or not, and eventually cancel the event.
  if (e.KeyChar.IsNumber)// if (!e.KeyChar.IsNumber)
    e.Handled = true;
}

void textBoxXX_KeyPress(object sender, KeyPressEventArgs e)
{
  // Check whether pressed character is number or not, and eventually cancel the event.
  if (e.KeyChar.IsNumber)// if (!e.KeyChar.IsNumber)
    e.Handled = true;
}