Change state of a button with multiple variables

Hi there, I just need some help.  I have a page with three variables text boxes on it.  The learner needs to click in each field and enter free-format text.  (This text then appears later in the module as part of a summary)  Once information has been entered into each field I want the 'Continue' button to appear.  Can someone offer their thoughts on how to do this.  I have tried a few different ways and can't get it to work. 

Can anyone also advise if there is a way to make the learner enter free-format text in a text field or am I restricted to 'click in the field' parameters.

I have attached the page from my file as an fyi.

Thanks in advance.