The averaging scheme means that the average income in box 1 is calculated over three consecutive calendar years - the averaging period. For each of the years of the averaging period, the tax on this average income is recalculated and compared with the tax actually paid in each of those years. If the recalculated tax is more than € 545 lower than the tax paid, an averaging refund will follow on request.
This refund can only be requested when all (final) assessments for the years of the averaging period have been imposed. For example, if you received a severance/transition payment in 2021 and the most favorable averaging period turns out to be 2021-2022-2023, then the averaging request can only be submitted in mid-2024 at the earliest.
An averaging request is therefore separate from the returns for the years of the averaging period and can be submitted separately. The request must be submitted within 3 years after the assessments for the years of the averaging period have become irrevocable. As it looks now (spring 2022), the averaging regulation will be abolished and 2022-2023-2024 will be the last period over which an average can be made.