Pretty sure you only want to define it once?

You create a volume, then map the mount into each service which needs to access it.

I am unsure whether there is performance differences? But the second one looks “wrong”

specific example on sharing data between containers https://www.baeldung.com/ops/docker-share-volume-multiple-containers

If you can get three, its likely you can get EE/Vodafone etc as well

Any suggestions

Use a different provider than Three. You will likely find you have much improved service