you have two ways to join adsense.
1. Go to "My account" --> Affiliate settings --> click "sign up" for the space given beside google adsense.
2. Directly go to http://www.google.com/adsense and fill in the required fields to get an account.
As you have already published three hubs, it will be better to join from hubpages. It may take upto 10 days to get approved. And once you get your account with adsense, you have to enter your id number to the space given here at hubpages, in "affiliate settings"
Hope that helps,