Is it possible or necessary to amend a tax return from more than 3 years ago?

Unless you tossed all that unsold inventory in the trash you have nothing to change.  Unsold inventory was converted to personal use so there is nothing more to deduct on the old return.

@dotterel -  the IRS works on a 'one sided coin'.

Once three tax years go by, you have to further right to a refund, but if the amendment would require you to pay more tax (not the issue here), you still owe it.

normally.....

for the 2018 tax year, the right to amend and get a refund expired on April 15, 2022

for the 2019 tax year, the right to amend and get a refund expires on July 15, 2023.