Unfortunately there is no support in our V3 API for tags. There are a few other areas where functionality differs between the API and the UI. You can read more about those here: http://developer.constantcontact.com/docs/contacts-api/api-gui-func.html
I do also want to mention that we are working on a new version of the API with the goal of greater compatibility with our modern product offerings and more complete support for their functionality. We don't yet have public dates that we can share, but if you're interested in being notified you can join our developer newsletter here: http://techblog.constantcontact.com/
If you have any questions, please let me know!