did you actually efile them or mail them in. if efile did you get acceptance email. if you mailed them, good luck, since apparently you did not use a service to track delivery. anyway go to IRS website and see if you can get transcripts.
if there are no transcripts for those years, the IRS never got your returns. hope you are due refunds because at this point if you owe, the penalties will be substantial
if you were due a refund for 2015, you out of luck. if you didn't file by 4/15/19
