title: extjs binding with formulas
body: <p>I'm trying to do textfiled binding in extjs with the following conditions.
  What  i'm doing wrong?</p>
  
  <ol>
  <li>Need to display three textfields where by default 2nd and 3rd textfields will be disabled.</li>
  <li>Second textfield should enable only when first field value is entered.</li>
  <li>third should be enabled only when first and second values are entered.</li>
  </ol>
  
  <p><a href="https://fiddle.sencha.com/#view/editor&amp;fiddle/363g" rel="nofollow noreferrer">the fiddle</a>.</p>