I just don't understand how three lines were shorted when the previous
> owners just moved out. Because they did previously have service!
> What's the best way to figure out where the short is?
It is possible that the previous owners shorted out the jacks so you couldn't make phone calls the day the house traded hands (when the calls might have been on their account). I happened to me once -- had to replace several phone jacks. In your shoes, I'd start by looking at all the phone jacks, and disconnecting any that didn't look right (after writing down how they were wired, of course).