It would probably increase performance since you have more endpoints you interact with, and the load is usually balanced between them.
It is a bit more safe, since a crash will wipe just a third of your data.
Not sure about cost, but if you have many changes in your topology in response to scaling etc. you'll get some moved errors, and you will have extra calls.
It has extra overhead in managing, but if you use a good client library, you wouldn't feel it, and you can also save costs using the latest valkey AZ affinity feature which supported by EC, and by valkey-glide client.