You can efile of file by mail your 2018 return. If you mail it, you can include 2017. You can't efile 2017.
If you mail them all at the same time, it is a good idea to use a separate thin envelope for each year's return and label each envelope with Tax year ___. Then mail them all in a one Priority mailing envelope.
https://accountants-community.intuit.com/questions/1288231-filing-past-years-returns-together-or-sep...
[Edited 4/14/2019 [7:35AM]
