"Validation" is about not giving a user the opportunity to make an error, as well as ensuring that user-entered (or run-time accessed, or pasted-in) data is in the correct format.
If you don't want the user to type in what is now your third TextBox, use a Label to show the answer.
One strategy:
1) initially enable only the first TextBox: when the user has entered a valid number, enable the second TextBox; when both have valid numbers, show the answer in the Label.
1a) you can use the 'Leave event of a TextBox to trigger validation; or you can sub-class TextBox to make a TextBox will accept only numbers, and/or decimal-point, backspace, key entries.
There is a heavier-duty facility for validation for Controls in WinForms: study that, and learn how to use it: [
^]. Example of validation with TextBox: [
^]