How to make calculation on text boxes

Question

I have three text boxes Textbox1 Textbox2 Textbox3.Now according to the project requirements if the user put an integer value 3 or high in box first then the 2nd box should show 1 and then this value should be show automatically in box third.and if the value in box first is less then 3 the the second box should show 0 or -. and then this value should show in box 3.
Please guide me .

my 2nd question is that what is the method to make calculation in text box.