Thank you for reaching out to Constant Contact's API Support.
You most likely used our API Tester page to generate your Access Token with our v2 API. When using our v3 API we are trying to be more security focused and so we are not displaying the Access Token on our API tester page, and they expire quicker.
You will need to go through the oAuth flow to generate your Access Token. You can automate this and after the initial login to allow access to the Constant Contact account there is no further interaction required.