more than one website
Entirely separate? No interaction at all?
If so, you could create three separate GA accounts or (recommended) you could use one GA account and have three separate properties created.
This would mean you'd have to create new properties (under the master account) for each site, making sure there were three separate UA codes generated.
Follow these instructions: https://support.google.com/analytics/answer/1042508?hl=en
If there is any cross linking then you will need to determine if that data makes sense to be recorded as a referral or the same session.
Hope that helps.