"My problem is that I also want to amend 2020 to show that it was repaid and thus back out the 2020 income that is 1/3 of the distribution."

But that's not what you are doing.

Maybe this will help:

The repayment is treated as falling under the Rollover provisions. There is no 60-day window, no "one every 12 months" limit, etc.

"2021 was filed without the 1/3 distribution as well. It will also need to be amended."

You don't report 1/3 now that all of it was repaid at once. You would not amend 2021 for 1/3, then amend again to remove this due to repayment in full.

Q7. May I repay a coronavirus-related distribution?

A7. In general, yes, you may repay all or part of the amount of a coronavirus-related distribution to an eligible retirement plan, provided that you complete the repayment within three years after the date that the distribution was received. If you repay a coronavirus-related distribution, the distribution will be treated as though it were repaid in a direct trustee-to-trustee transfer so that you do not owe federal income tax on the distribution.

If, for example, you receive a coronavirus-related distribution in 2020, you choose to include the distribution amount in income over a 3-year period (2020, 2021, and 2022), and you choose to repay the full amount to an eligible retirement plan in 2022, you may file amended federal income tax returns for 2020 and 2021 to claim a refund of the tax attributable to the amount of the distribution that you included in income for those years, and you will not be required to include any amount in income in 2022. See sections 4.D, 4.E, and 4.F of Notice 2005-92 for additional examples.

Obviously, your taxpayer cannot claim a refund for taxes never paid.