Amending tax return

Hi, my question is when is the last day I can amend my 2018 and 2019 tax return. So I don’t owe any taxes for those years even when I amend it. I filed my 2018 tax at February 2019 and I filed my 2019 tax at April 2020. So when is the deadline that I can amend them? My research says that there’s only 2 year or 3 year after filing for amending a tax return?

thank you