welington

 if ((maskedTextBox1.Text.Length > 0) && (maskedTextBox2.Text.Length > 0) && (maskedTextBox3.Text.Length > 0)) ;
            {
                double n1 = Convert.ToDouble(maskedTextBox1.Text);

                double n2 = Convert.ToDouble(maskedTextBox2.Text);

                double n3 = Convert.ToDouble(maskedTextBox3.Text);

                textBox6.Text = Convert.ToString((n1 + n2 + n3) / 3);

                double media = Convert.ToDouble(textBox6.Text);


                if (media > 5)
                {

                    textBox5.Text = "Aprovado";
                }
                else
                {
                    textBox5.Text = "Reprovado";


                }
            }
        }

https://snag.gy/ZBK0DQ.jpg