To complete and file tax returns for prior tax years, 2015 thru 2017, using TurboTax you would need to purchase, download and install on a persona computer each tax year needed from this TurboTax website - https://turbotax.intuit.com/personal-taxes/past-years-products/
Prior year tax returns can only be printed and mailed, they cannot be e-filed.
When mailing multiple tax years use separate envelopes for each tax year being mailed.