you can't amend any return prior to 2017. those are closed years. what a 3115 does is you tell the IRS you messed up in claiming depreciation. then the IRS lets you take the deduction you should have taken even for those closed years. see a pro. if you mess up what could eventually happen is all that depreciation that you should have taken but didn't. well, when you sell you have to treat the sale as if you took all that depreciation which means you'll be paying taxes for a deduction you never got the benefit of.
even if you were to amend 2017 and 2018, without the 3115 you'll lose out on the depreciation for 2014 through 2016, but will have to pay taxes when you sell as stated above.
