Here they published the https://github.com/brandonseydel/MailChimp.Net api 3.0, it is live now. But they didn't say anything about how to use batch to upload many members in one request.
I have searched in stackoverflow and also google, they either implement using php or said it was not supported yet. such as from this topic (MailChimp API 3.0 batch/bulk subscribe), was from 2015. As a year passed, just wonder anyone know how can I send multiple members (email addresses) to a list in one request using the .net api 3.0