Retirement tax questions

If you received a distribution in 2020 and elect to pay over the 3 year period but didn't split the income equally?  Can I split the remaining amount in 2021 and 2022 or do I need to amend the 2020 taxes?  Example:  Received $60K and in 2020 only claimed $6,666 as income because I divided my amount wrong (60000/3=20000 and then 20000/3=$6666) so I put $6666 for 2020.   The calculation should have been $20K each year which means I was short $13334 for 2020.  Now my total amount due is $13334 in 2020, $20000 in 2021 and $20000 in 2022 which is a total due of $41,334 split over the next 2 years or do I have to amend and fix 2020 and have them all equally at $20K per year?