Hi Katherine,
Was it "insta-booked" or did they make a request and then you approved it?
If the former, I'm not sure what you can do at this stage......and if the latter I suppose the only way to avoid it is to clarify the pet count before approving but I get that this takes time.
Totally agree that they should have an option to mark how many pets you permit (or perhaps even what types / sizes...)
Is it still a while before she arrives? If so, and she's a reasonable person, it might be best to cancel it if you're not comfortable with 3 large dogs coming........
Erik