HTML:
<input type='radio' name='r1' value="NE" />
<input type='radio' name='r1' value="NW" />
<input type='radio' name='r1' value="SE" />
<input type='textbox' id="txt1" name='txt1' value="NE"/>
<input type='textbox' id="txt2" name='txt2' value="NW"/>
<input type='textbox' id="txt3" name='txt3' value="SE"/>
JavaScript:
// First assign onclick handler to your radio buttons
// Note - this is just a demo. You should encapsulate 
// this loop in a page onload initialization method.
var radios = document.forms[0].elements["r1"];
for(var i = 0; i < radios.length; i++) {
   radios[i].onclick = DisableTextBoxes;
}
// Disable textboxes based on respective radio click
function DisableTextBoxes(){    
  var textboxes = [
     document.getElementById("txt1"),
     document.getElementById("txt2"),
     document.getElementById("txt3")]
  for(var i = 0; i < radios.length; i++) {
     if(radios[i].checked) {
        textboxes[i].disabled = false;
     } else {
        textboxes[i].disabled = true;
     }
  }
}
A more thorough explanation can be found here.