@Marvin is running prefect workflows with version 3 compatible with my self-hosted server being version 2?

No, running Prefect workflows with version 3 is not compatible with a self-hosted server running version 2. Prefect aims to maintain backwards compatibility with old clients for each server release, but new clients, like Prefect 3, may expect the server to support functionality that it does not include. It's recommended that all clients are the same version as the server, or older. For instance, a client on a newer version can be used with a server on a slightly older version within the same major version, but not the other way around. For detailed information, please refer to the Prefect documentation: https://docs.prefect.io/latest/contributing/versioning/