That's a tough one because like you said, the insurance carriers will only mail to one address for all locations. The only other way to split it up is if she would get a separate name and EIN/Tax ID # for the other two locations, which she probably does not want to do.
The PO Box might work, depending on who separates things, but the eobs will contain billing for both on the same remit. Also, it will take some work to switch the correspondence address.
Good luck!
Michele