Tax gain by selling under construction property

Yes, there shall be long term capital gain on the sales of this under construction property as the property was held for more than 3 years even when the assessee was not in physical possession. The sale agreement shall also factor in any outstanding payment to be made to the builder. And separate formalities to be followed in case there is a housing loan on this. You may avail exemption u/s 54 or 54EC. 

Agree with Mihir ji,

However under such case a question will arise what would be the date of acquisition as the possession of the property haven't transferred...

For a flat that is under construction, when does your holding period begin and how is it computed?

For this purpose, it is important to note that there is a difference between a flat which is still under construction and the possession has not been given to you by the builder, and a flat which is ready and the possession has already been taken by you.

Though both are capital assets, in the first case the asset is not the flat (as it is not yet complete and, therefore, not in existence) but the right to acquire a flat, while in the second case the asset is the flat itself. You, therefore, need to determine what is it that you are selling—the right to acquire the flat or the flat itself—to be able to determine the date of acquisition.

If the Property is being constructed and the payment is made in installments,

Then there will be proportionate capital gain of Short term and Long term on installments basis...