Hi. My name is***** Thank you for giving this little one a good new home. Has she been to your vet yet for any health check? What food is she eating normally?
That is great that the other two cats are doing well at this time. Hopefully they are adjusting to her addition to the family well. In a situation like this with a new kitten having loose bowls this persistently, intestinal parasitism / coccidia / giardia would all be very big suspicions. What would be a great thing to do here is to get her to your vet to have her checked over with a physical exam. Here at 9 weeks of age, she can be started on her vaccine series. They can at this time also do a fecal evaluation to see if any of the things mentioned prior are there and need to be specifically treated. If they are, they can do that. Some cats can have dietary sensitivity and certain foods just give them loose stools.
Until you get to the vet, I would continue to make sure that she's eating well, playing well, being a kitten and being well loved. I wouldn't try to give her anything specific for the stools. You could try to change her diet to another brand of kitten food to see if that helps.
There are over the counter dewormer medications out there. IF one was obtained and given at the right dosage, it may help if say round worms were the issue.
Make sure on any over the counter medications, the weight and age requirements are met.
I'm hoping for the best for this little one! Take care.