To calculate the sum in PHP use a for loop to go through each database row returned, and add the value to a variable.
public function getSubSum() {
$services = ServiceProvided::get()
->filter('InvoiceObjectID', $this->ID)
->limit(3, 0);
$sum = 0;
foreach($services as $service) {
$sum += $service->Sum;
};
return $sum;
}