How do I file for 3 years back in this app. I don't see an option to file for multiple years. This is my first time filing my own taxes. In the past I hv used a service.

The TurboTax online editions are only for tax year 2019.

To complete prior tax years using TurboTax, 2016 thru 2018, you would need to purchase, download and install on a personal computer one of the desktop editions for each tax year you need to file 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.  Use a separate envelope for each tax year being mailed.