You are obviously a good Mom, I mean three Greyhounds in one house? Heaven! How do you ever bear to go out the door without them?!

The original lapse in house training was almost certainly due to the stress of having another dog in the house upsetting the pack structure.
I would also agree that a trip to the vet would be a good idea, especially as it is a urination problem now.
If there is no health problem then going back to basics with the house training may help although I suspect it has more to do with the pack structure changing in some way. Has the dog been less bold, showed more submission to the other dogs than usual? It may be very subtle.

Can you shut doors to other rooms until the problem is sorted?

Keep an eye out for signs of stress and how the dogs are interacting with each other. Has your routine changed at all? Very small changes can upset a sensitive dog, especially if she was made insecure by another dog in the house.

Is she spayed? Coming into season?

The vet is the first thing to check up on though - good luck with her and let us know how you get on and bless you for taking track dogs on - aren't they a dream?!