Limit number of checkboxes checked inside ng-repeat

So I have javascript code that checks how much checkboxes is checked. It works well except that when I hit 4 checkboxes it disables all checkboxes not only unchecked (you can see it on image below). Thank you!

JAVASCRIPT

$scope.checkedb= function (unit, device, unitt){
  unitt.showcb = !unitt.showcb;

 numOfCh = 0;
  for (var i = 0 ; i < unit.length; i ++){
    if (unit[i].showcb == true){
      numOfCh ++;
    }

    if (numOfCh > 3){
      device.selectedf = true;
    }
  }
}

HTML

<div ng-repeat="unit in data[unitIndex].units" ng-if="unit.users_ID == tvw.id">

        <td class="footable-visible" > 

           <input type="checkbox" ng-disabled="device.selectedf" ng-click="checkedb(data[unitIndex].units, device, unit)">

        </td> 
</div> 

as you can see the checked checkboxes are also disabled.