If you still have credit remaining after properly calculating the credit that applied to 2013, 2014 and 2015 but you failed to claim, that remaining credit can be applied to the 2016 through 2018 tax years that are still open for amendment to claim a refund by amending those tax returns. However, the portion of the credit that was applicable to 2013 through 2015 is lost. You'll have to do pro forma 2013, 2014 and 2015 amendments to see if you have any credit left that carried into 2016.
