Hi,
Your answers are as below:
1) You will not be eligible for exemption u/s 54F as you already own 2 Residential Flats. However, if the property in Lucknow is Residential you can claim exemption u/s 54 if you acquire another residential property out of the amount of Capital Gains earned through sale of Lucknow Property. If the property at Lucknow is other than residential, you will not be eligible for any exemption u/s 54 also. As such, in that case, only Plan B shall be valid.
2) (a) If you receive the sale proceeds in May 2018, you will have to pay Tax before 15.09.2018 in order to avoid any tax on late payment.
(b) Due date for filing of ITR shall be 31.07.2019
3) The indexed cost of acquisition for you shall be 1,54,367 x (CII for FY 2018-19) / 100. CII for 2018-19 not yet declared.
Regards,