Date of acquisition of under construction property sec54

Hello,

I purchased flat A in year 1995 and sold in Oct 2011 

I bought ready to move flat B from the LTCG in year Sept 2013. (I had put the money in CGAS during Oct 2011-Sept2013)

Now i will be selling the flat B in Oct 2016 (3 years over)

My Query:

If i buy a ready to move flat C from the LTCG of flat B in  Nov 2016 then the 3 year lock in period will start from Nov 2016.

But if i buy under construction flat say in Nov 2016 and also do the registration and also spend all the money in the gains then what wll be my start date for the 3 year lock in period

- Will it start from Nov 2016 - the registration date i.e will be able to sell it in Nov 2019   OR

- Will it start from the date of possession say April 2018 ?

In short does the 3 year lock in period begin from date of registration or date of possession ?