How to make website users go through pages in order

I am working on a simple website that requires users to enter information in order over 3 different pages. So they need to go from page A to B to C. How can I stop them from typing in http://ift.tt/2wT7LMR, which skips page A?

Also, I'm not even sure what you call pages that must be visited in order, so any suggestions for google search terms on this also appreciated.