This has happened in the past ... you will get a 1099-R for the entire 2020 distribution and the withholding next January which you will enter as printed THEN the program will ask you if you want to split the distribution over 3 tax years and the program will complete the correct forms for you. It will show up on the 1040 form on line 4a in full but only the taxable portion will be on 4b. All the withholdings will be reported on the 2020 return only ... then if you will owe taxes in 21 & 22 due to the distribution being spread out then you need to make arrangements to either withhold taxes from some other income, pay in estimated payments OR take some of the excess 2020 refund and apply it to the 2021 return.
