<form name='myform' action='index.php'>

<input type='hidden' name='value1if3boxesticked' value='' />

<input type='checkbox' name='box1' value='1' onClick="myFunction(this)" />
<input type='checkbox' name='box2' value='1' onClick="myFunction(this) />
<input type='checkbox' name='box3' value='1' onClick="myFunction(this) />
... to 10
</form>

var ticked=0
function myFunction(what)
{
  if (what.checked) ticked++; else ticked--
  what.form.value1if3boxesticked.value=ticked>3?'something:''
}

function myFunction()
{
  var ticked=0
  for (var i=0,l=document.myform.elements.length;i<l;i++)
    if (document.myform.elements[i].type=='checkbox')
      if (document.myform.elements[i].checked) ticked++
  document.myform.value1if3boxesticked.value=ticked>=3?'something':''
}
window.onload=myFunction

<script>
        
//these all tick the pdp and redflag checkboxes based upon the value in the dropdown for that row
function FlagTicked1(control) {
document.myForm.1pdp.checked = (control.value=="2")? true : false;
document.myForm.1redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked2(control) {
document.myForm.2pdp.checked = (control.value=="2")? true : false;
document.myForm.2redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked3(control) {
document.myForm.3pdp.checked = (control.value=="2")? true : false;
document.myForm.3redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked4(control) {
document.myForm.4pdp.checked = (control.value=="2")? true : false;
document.myForm.4redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked5(control) {
document.myForm.5pdp.checked = (control.value=="2")? true : false;
document.myForm.5redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked6(control) {
document.myForm.6pdp.checked = (control.value=="2")? true : false;
document.myForm.6redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked7(control) {
document.myForm.7pdp.checked = (control.value=="2")? true : false;
document.myForm.7redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked8(control) {
document.myForm.8pdp.checked = (control.value=="2")? true : false;
document.myForm.8redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked9(control) {
document.myForm.9pdp.checked = (control.value=="2")? true : false;
document.myForm.9redflag.checked = (control.value=="1")? true : false;
}
function FlagTicked10(control) {
document.myForm.10pdp.checked = (control.value=="2")? true : false;
document.myForm.10redflag.checked = (control.value=="1")? true : false;
}
        
</script>

<form method='post' action='form.php' id='myForm' name='myForm'>
        
<input type='hidden' name='redflagnotify' value='' /> <!-- this needs to be value='1' if there are 3 redflags checked -->

<table>
<tr>
<th>Code</th>
<th>Question</th>
<th>Score</th>
<th>Witnessed</th>
<th>PDP</th>
<th>Red Flag</th>
</tr>

<tr>    
<td>111</td>    
<td>Question 1</td>
<td>
<select name='1score' onchange="FlagTicked1(this)">
<option  value=''>- Please select -</option>
<option selected="selected" value='3'>3</option>
<option  value='2'>2</option>
<option  value='1'>1</option>
</select>    
</td>    
<td><input type='checkbox' name='1witnessed' value='1' /></td>        
<td><input type='checkbox' name='1pdp' value='1' onclick="return false" onkeydown="return false" class='disabled'  /></td>    
<td><input type='checkbox' name='1redflag' value='1' onclick="return false" onkeydown="return false" class='disabled'  /></td>
</tr>

<tr>    
<td>222</td>    
<td>Question 2</td>
<td>
<select name='2score' onchange="FlagTicked2(this)">
<option  value=''>- Please select -</option>
<option selected="selected" value='3'>3</option>
<option  value='2'>2</option>
<option  value='1'>1</option>
</select>    
</td>    
<td><input type='checkbox' name='2witnessed' value='1' /></td>        
<td><input type='checkbox' name='2pdp' value='1' onclick="return false" onkeydown="return false" class='disabled'  /></td>    
<td><input type='checkbox' name='2redflag' value='1' onclick="return false" onkeydown="return false" class='disabled'  /></td>
</tr>

<tr>    
<td>333</td>    
<td>Question 3</td>
<td>
<select name='3score' onchange="FlagTicked3(this)">
<option  value=''>- Please select -</option>
<option selected="selected" value='3'>3</option>
<option  value='2'>2</option>
<option  value='1'>1</option>
</select>    
</td>    
<td><input type='checkbox' name='3witnessed' value='1' /></td>        
<td><input type='checkbox' name='3pdp' value='1' onclick="return false" onkeydown="return false" class='disabled'  /></td>    
<td><input type='checkbox' name='3redflag' value='1' onclick="return false" onkeydown="return false" class='disabled'  /></td>
</tr>

<!-- etc etc up to 10 -->

</table>

function FlagTicked(control)
{
  var nr=parseInt(control.name,10)
  document.myForm.elements[nr+'pdp'].checked = control.value=="2";
  document.myForm.elements[nr+'redflag'].checked = control.value=="1";

  var ticked=0
  for (var i=0,l=document.myForm.elements.length;i<l;i++)
    if (document.myForm.elements[i].type=='checkbox'
    && document.myForm.elements[i].name.match(/^\d+redflag$/)
    && document.myForm.elements[i].checked)
      ticked++
  document.myForm.redflagnotify.value=ticked>=3?'1':''
}