IT return filing for Capital gains

My uncle is 85 years old and I manage his equity portfolio. There were no 'taxable' capital gains for past two financial years (2018-19 and 2019-20), gains were within the income tax limit. We had not filed his ITR for the two years as there was NIL tax. There is no other income.

For FY 2020-21 I sold shares and the capital gains arising are above the income tax limit and taxable.

So can we file ITR only for FY 2020-21 or it is mandatory to file returns for past two financial years also? Please guide.