Since you can amend a tax return for the current tax year and three years back, you have no need or requirement to file the 3115. You need to amend the 2017 and 2018 tax return. Note that you *MUST* amend starting with the oldest return first. Additionally, when you amend the 2018 return you can not import your carry forward data from the amended 2017 tax return. It's physically impossible to do so. Therefore you will have to manually enter the carry forwards from the 2017 return, into the 2018 return when you amend it. It is also imperative that the carry foward amounts be correct. 99.9999% of people in your situation get the amounts wrong and end up getting audited anyway.
So get the 2017 tax return amended and once you've done that *PRINT* *EVERYTHING* (not just the forms required for filing) and post back for guidance on the 2018 amending process.
Take note that an amended return can not be e-filed. The IRS Says so. You have to print, sign and mail it to the IRS. Each individual amended return has to be mailed to the IRS in a separate envelope too. But you don't mail anything to anyone until you have completed *everything* and are satisfied with the results.
For amending guidance of a prior year return, see https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/26768 I know it refers to 2016. But the basic guidance still applies.
Again, note that you need to complete each return in order, starting with the oldest first. You really can't even start your 2019 return until you've amended the 2018 return. That's the only possible way to get the correct numbers into the 2019 return, from the amended 2018 return.
