If it was a 2021 contribution *for* 2020 then it had to be reported on your 2020 tax return as a non-deductive contribution on a 2020 8606 form for you to apply it to a distribution/conversion.
You need to amend 2020 and add the non-deductible contribution in the IRA contributions section and mark it non-deductible to produce the 2020 8606 form.
Then on 2021 tax in the 1099-R interview you say you had $3,000 of prior year non-deductible contributions that comes form line 14 on the 2020 8606 form.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
