To complete and file a 2016 and 2017 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the desktop editions for each tax year from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
2016 and 2017 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. Consider using a tracking service from the US Post Office so you will get verification that the mailed tax returns were received.
For tax year 2018 you can use either the TurboTax online or desktop editions. A 2018 tax return can be e-filed up through October 15, 2019. After that date the 2018 tax return can only be printed and mailed.
TurboTax 2018 online editions - https://turbotax.intuit.com/personal-taxes/online/
TurboTax 2018 desktop CD/Download editions - https://turbotax.intuit.com/personal-taxes/cd-download/