We can't tell you what happened in the past, and we can't see your returns.
Looking at form 8606 for 2021, line 2 (prior basis) would be zero and line 14 (new basis) would be $6000.
For 2022, line 2 (prior basis) would be $6000 and line 14 (new basis) would be $12000.
For 2023, line 2 (prior basis) should be $12,000 (based on your history).
If you didn't do any conversions and are only reporting basis, and it doesn't change your tax owed or refunded, you can file an amended form 8606 for 2021 and 2022 by themselves without filing a whole amended return. Be sure to use the correct version for each year.
Start by checking your tax transcript for 2021 online to see what the IRS has on file for you.
https://www.irs.gov/individuals/get-transcript
For 2023, you can enter that you have a prior basis of $12,000 without having to amend first. But make sure you keep records of your contributions and your form 8606. You need to keep that for as long as you live (or until the IRAs are emptied out) because the IRS will not keep track of your basis, if you want to prove that a portion of your retirement withdrawals are non-taxable, the burden of proof is on you.
