unless you use a professional you'll have to manually prepare returns for 2013-2015 since software is no longer available. you can find this software in the web, but it is not updateable and likely to produce an incorrect return. if you are due a refund for those years, you won't get it because you have filed past the statute of limitation. if you owe, expect to pay penalties and interest besides the taxes.
there may be reasons to file those returns even if there is an overpayment (which you won't get) such as loss carryforwards which will be lost if you don't file.
you have to buy separate software for each year 2016 - 2018. (2018 can no longer be prepared online) make sure your computer can run the software and be sure to get the right version depending on whether you have a PC or MAC.
windows:
https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...
mac:
https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...
start with the earliest year. this will allow transfer of certain data to the next year.
