If you have not yet filed your 2014 and 2015 tax returns, it is too late to "waive the carryback" to carry the NOL to 2015 and 2016. You must now amend your 2012 and 2013 tax returns to claim the NOLs.
Yes, you can still amend your 2012 and 2013 tax returns to claim the NOLs. Because the 2014 and 2015 tax returns (the years of the NOLs) are still within the 3-year window, you can still amend 2012 and 2013 to claim the NOLs.
You probably already know this, but some people misunderstand what a Net Operating Loss (NOL) means. It does not just mean your business had a loss, but your total income on your tax return is negative (Line 41 of your Form 1040 is negative) due to the business loss. If your business had a loss but you still had positive income on the tax return (Line 41 is a positive number), you would not have a NOL.
