You’ll be using the APIs of the respective services (Google, Facebook, Twitter) or may be OpenID if you plan to add that as well.
Some links:
http://code.google.com/apis/accounts/docs/OpenID.html
https://developers.facebook.com/docs/authentication/
http://dev.twitter.com/pages/auth
Also take a look here.
There are several similar questions already present on SO, check the related questions on the right.