Can I file amendments to my returns for 2018, 2019, and 2020 with a correct cost of property instead?
No. What you have is an accounting error, which is something bigger than just a math error. you can not amend returns. You *have* to do the 3115 to correct the accounting error.
Generally, unless otherwise provided, a taxpayer must secure the IRS's consent before changing its accounting method. To obtain the IRS's consent, taxpayers file Form 3115, Application for Change in Accounting Method. Even when the IRS's consent is not required, taxpayers must file Form 3115. This form is not simple by any stretch. Trying to do it yourself and getting it wrong will just turn your current molehill into a mountain. you *need* professional help. Especially if your state taxes personal income, because you've been taking incorrect depreciation on your state return also.
