If there is a carryback in 2017, you can use Form 1045. The remaining will be carried forward onto the 2018 return. See https://www.irs.gov/pub/irs-prior/i1045--2017.pdf and https://www.irs.gov/pub/irs-prior/f1045--2017.pdf
If the year 2017 was your loss year, you can carry back two years. Instead of filing 1040X for 2015 and 2016, you can use 2017 Form1045 to recalculate the tax due to the loss for those two years. If there is any carryforward left, you would use it on your 2018 taxes. It is faster to process this way than an amendment 1040X.
Update: For most taxpayers, NOLs arising in tax years ending after 2017 can only be carried forward. The 2-year carryback rule in effect before 2018, generally, does not apply to NOLs arising in tax years ending after December 31, 2017. Exceptions apply to certain farming losses and NOLs of insurance companies other than a life insurance company. For details: click here: https://www.irs.gov/newsroom/net-operating-losses
[Edited 3/9/2020 | 5:12PM]
