Yes, you can install Omnisphere 3 normally to your computer if you have a previous version of Omnisphere (version 1 or 2) already installed.
Since, Omnisphere 3 is fully backwards compatible, your existing User data and user libraries will be preserved.
Additionally, Omnisphere 3 it will replace the previous version of Omnisphere in your existing projects and retain the original settings for the track.
If you're installing with a USB, you can find instructions here:
If you're installing with the download, you can find instructions here: