I prefer user sessions for this, somtimes combined with cookies, which temporarily store the form input data and allow you to ccall it over multiple pages and are set to self destruct after the form has been submitted.
You can do it other ways; passing the form variables from one to another form page via hidden form fields or by populating a database with the form submission and calling it back in the event of an error in your form data, identified by a thorough form handler script.
This is complex to get into, so don't expect a line or two of code to solve the issue.
Try this link from the excellent Zend site to give you an idea of one method: click here
Frankly, for only three form fields this and user sessions could be vastly overgunning things. It's workable though.