Thank you for reaching out to Constant Contact API Developer Support.
Unfortunately we do not have any SDK libraries for our new V3 API at this time. Right now all of our focus is on finishing additional endpoints to expand on the feature set of the V3 API. You can find our complete V3 API documentation at https://v3.developer.constantcontact.com/ and the API Reference section will have all of the necessary schemas you can expect. You can make calls to our API in .NET using something like HttpClient with the information showing in our documentation.
Please let me know if you have any other questions!
Regards, David Bornstein Tier II API Support Engineer