Thank you for reaching out to Constant Contact API Developer Support, my name is Courtney. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Here is the documentation on how to migrate your existing integration to V3 with minimal user impact:
Data Migration Overview
Additionally, here are the steps for the authentication process in V3. After you complete the OAuth2 server flow, Constant Contact returns an access token and refresh token. You can use access tokens to make requests using the V3 API. You can use refresh tokens to obtain new access tokens without any user input. This ensures that your users only need to authenticate once.
V3 API OAuth2.0 Server Flow
If you are still having trouble after reviewing the documentation, we may need to know about what specific issues that you are running into, and some more details about the application that you are trying to migrate in order to better assist you.
For further assistance with this, please feel free to either reply within this thread, or contact us directly via email at firstname.lastname@example.org, and reference case number 28612783.