I need to file my 2017 and 2016 taxes, I filled the 2018 on February 10,2018 through another tax service. How do I file the other years through free turbo tax?

The TurboTax online editions which include the Free edition are only for tax year 2018.  There is no Free edition for prior tax years.

For prior tax years (2015 thru 2017) if you choose to use TurboTax then you would need to purchase, download and install on a personal computer one of the desktop editions for each tax year required from this 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.

Or you can get free forms, schedules, publications and instructions for a prior year federal tax return from this IRS website - https://apps.irs.gov/app/picklist/list/priorFormPublication.html