بسم الله الرحمن الرحيم
Zakah will be wajib on PF amount only when it comes into your possession. And in case of keeping for additional three years the extra amount which you will be receiving will not be counted as interest.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.