In TurboTax you'll enter the entire distribution just as it is reported on the Form 1099-R that you'll be receiving from the 401(k) plan. Once Form 8915-E is implemented in TurboTax, TurboTax will prepare this form to allow for splitting the income over three years.
As SteamTrain said, any tax withholding from this distribution will be credited on your 2020 tax return. If if the result is a refund, you have the option to apply that refund to your 2021 taxes equivalent to a Q1 estimated tax payment.
