Filing last 3 year ITR returns in this year

1. The last date for filing the belated return as per sec 139(4) it should have been filed before the end of the assessment year or before the end of the assessment, whichever is earlier. 
2. In your case return for the year, 2016-17 and 2017-18 ends on 31.03.2018 and 31.03.2019 respectively. Hence belated return cannot be filed in the given case. 
3. However, for the FY 2018-19 return can be filed till 31.03.2020 with a fine of Rs. 1,000 in case if the total income is below Rs. 5 lakh. And if the total income is above Rs. 5 lakhs fine of Rs. 5,000 up to 31st Dec 2019 and Rs. 10,000 if the return is filed after that period. 
Please correct me if the above solution has an alternative view.