Any depreciation you claimed in past years just adds to the unrecapture section 1250 gains tax you owe in the year you sell the property. So filing the 3115 for 2017 would reduce you tax burden for 2017 but it would increase your burden for 2021 by the same amount. It wouldn't be worth the effort to amend your 2017 return since it doesn't change the total tax you owe between 2017 and 2021.
https://www.investopedia.com/terms/u/unrecaptured-1250-gain.asp
