Oh no---did you use the current online program to prepare and e-file 2016? The online program is ONLY for 2018. If you used it for 2016 income and e-filed it and it was accepted, you have made a really big mistake. You filed 2016 as if it was 2018 income. That means after the IRS processes that return, you will have to amend it and use the 1040X to enter your 2018 income.
As for 2016 and 2017, you should have used the downloaded desktop software for those tax years. Each year has its own software written for the tax laws in effect for the specific year.
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)
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.
Please post back to this thread and tell us what you did so we can advise further if we have misunderstood what you did.