If you are intending to sell the flat soon after taking possession, it is better to carry out the sale while the flat is still under construction.
The flat under-construction and the possession has not been given to you by the builder is not flat but the right to purchase the flat.
When you take possession of the flat which you have agreed to purchase, the right to purchase the flat gets converted into the flat.
Therefore, if you are selling the flat after taking possession of the flat, the period of three years starts from the date of taking possession of the flat.
The Property is regarded as a capital asset and any gains arising from the sale of property is assessed under the head ‘Capital Gains’ as per the Income Tax Act.
The capital gain should be invested in the purchase of another house either one year before or within two years from the date of transfer, or used for construction of a house within three years of the date of transfer.