You would use them the same way you've used your other two.
1. Install the software - http://support.presonus.com/hc/en-us/articles/210040323
2. Activate it - http://support.presonus.com/hc/en-us/articles/210049863
3. See Below:
If you need to install Studio One 3 on a second computer or you need to re-install Studio One 3 due to hardware changes on your computer, you can get the latest version of the software in your account under the My Products>Software section of your account. This will also insure that you are installing the latest build of the software as the Studio One installer located in your user account will always be the latest build available for download.
If for any reason you need instructions on how to obtain that download file, please see the following knowledge base article for assistance:
You can also manage your activations from your user account if needed. Simply click on the link with in the registered software section entitled "Manage Activations" and remove any activations that are no longer active.