How does PDU's during 3 Year Cycle work.

Lets say if i collect 60 PDU's in my first year, are they still counted towards 3 years, or once i report then i have to start collecting more PDU's for following year?

Can someone please explain?

Also if i have collected more than 60 PDU's during a 3 year cycle, does the extra get rollover for following 3 year cycle?