I got to ask, why would you send a snake to be boarded if your only gone for three weeks? All snakes can live much much longer without food and as for water, a larger bowl would probably do it. And I'm sure you could of found one person to top up the water.
I'm not tring to be an *** but putting a pet snake into a petshop will cause more troubles than its worth! Petshops have a lot if not most wild cought animals in them, and as you now know first hand don't always look after them as they should