Try:
textBox3.Text = (Convert.ToInt32(textBox1.Text) * Convert.ToInt32(textBox2.Text)).ToString();
C# requires:
1) Correct case names - it is case sensitive so
TextBox1
is not the same as
textBox1
2) Matched brackets!
3) Unlike VB, C# does not do implicit type conversion - you have to change the type yourself.