the requirement for a year of an NOL is to make an election to forgo the NOL carryback. The election must be made on a timely filed return. so the 2015 NOL has to be carried back to 2013 (or maybe 2012 -see below for qualified small business NOL) any remaining would be carried forward. if nothing remaining it is lost because the time period for filing a 2015 NOL c/b claim has expired (had to be filed by 4/15/2019).
you may qualify for a 3-year c/b if the loss was attributable to a qualified small business
Qualified small business.
A qualified small business is a sole proprietorship or a partnership that has average annual gross receipts
(reduced by returns and allowances) of $5 million or less during the 3-year period ending with
the tax year of the NOL. If the business did not
exist for this entire 3-year period, use the period
the business was in existence.
An eligible loss does not include a farming
loss or a qualified disaster loss.
https://www.irs.gov/pub/irs-prior/p536--2015.pdf
the same would apply to 2016 - carryback to 2014 (or 2013) any remaining gets carried forward. the NOL rules were the same.
i would suggest you seek pro help in preparing those missing returns and determining whether you have any NOL's left to carry forward to 2018 and beyond
