time limit for assessment

As per section 153,the time limit for making assessment under section 143(3) is
1.Within 21 months from the end of assessment year in which the income was first assessable
2. 18 months from the end of AY in which the income was first assessable
3.12 months from the end of the AY in which the income was first assessable

for assessment year 2017-18 time limit 21 months or 18 months , Please calaify

if case selected u/s143(3) due date is 31st dec