I thought it might be too late to prepare the Form 1139, however, I am confused about how I would amend the return for the carryback.

2016 Profit

2017 Profit

2018 Loss 

Since I can't go back and apply the 2018 loss to 2016 via the Form 1139 how can I carry back via an amended return?

This is a little new to me so I am trying to understand what to do,

Thank you.