Thank you for reaching out to Constant Contact API Developer Support.
We're still finishing up new endpoints for our V3 API, and we aren't planning on building out any SDK libraries until they are finished at least. So it will be some time before any would be available. However, our V3 API is just a RESTful API, so there are various ways to make HTTP calls depending on your needs. You can find our full V3 API documentation at https://v3.developer.constantcontact.com/api_guide/getting_started.html
Please let us know if you have any other questions!
Tier II API Support Engineer