//replace the array elements with your exact checkbox names
$checkboxes = array('my-first-checkbox','another-checkbox','last-checkbox');
 
$selected = false;
foreach($checkboxes as $cbox){
	if(isset($_POST['form'][$cbox])){
		$selected = true;
		break;
	}
}
if($selected === false){
	//also replace the checkbox name here with one of your checkboxes
	$invalid[] = RSFormProHelper::getComponentId("my-first-checkbox");
	//this will be the checkbox that invalidates
}