I haven’t filed for 2016 2017 or 2018 I need to do so. How do I file separate years

The current online software sis ONLY for 2018.   You cannot use it for any other year.  Only a 2018 return can be e-filed; other years must be printed, signed and mailed in SEPARATE envelopes.  Never combine tax years in the same envelope.

To file a return for a prior tax year

If you need to prepare a return for 2015, 2016, or 2017 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.