Date formats

what is the date format for last 3 hours ?

I read the docs, it was mentioned that I can use “Today”, “Current week”, “Current month”, or “Last 7 days”, “Last 30 days”, “Yesterday”, etc

I need the reports of last 3 hours

to limit per hour, you should use “segmentation” feature and restrict where visitServerHour > 13 and visitServerHour < 17
http://piwik.org/docs/analytics-api/segmentation/#toc-list-of-segments