The math is not working for me. If you filed 2020, 2021, and 2022 with 1/3 each, there is no 1/3 left for 2023. You do want to claim the repayment. You can choose based on the amount repaid and the time you want to put into it. If you repaid less than 1/3, you could just amend the last return, 2022. The more you paid back, the more returns to amend. The deadline for a refund from 2020 expires April 15 so start with that one if you are doing them all.
When you fill out the 1040x, you will enter the description for paid back loan. I doubt you have any official IRS paperwork to go with the forms.
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