You can add as many Jetpack sites as you want to a single WordPress.com account. Once you've done so, you will be able to access your stats in the 3 dashboards, or directly on WordPress.com. A dropdown menu will appear there and will allow you to switch between the 3 sites.
To connect a site to your WordPress.com account, go to the Jetpack menu in your dashboard, and click on the Connect button.
Let me know if it helps.