I need to file 2016, 2017, 2018 taxes.

The tax return for each year must be completed with the program for that year only. You must buy, download and install the program for each tax year. Here is a link: https://turbotax.intuit.com/personal-taxes/past-years-products/

Prior year returns must be printed and mailed. They can't be efiled. You must print and mail them yourself. Mail the return for each year in a separate mailer so that the IRS doesn't overlook a return included in the mailer with another return. Follow the filing instructions that print with each tax return for what to include and where to mail it. 

As an alternative for simple tax returns, you could consider downloading the forms for each tax year from the IRS web site and manually completing them. There are instructions on the web site to help you.

Here is a link to find prior year forms: 

If you are getting a refund for 2016, you have until July 15, 2020 to file a return and still get the refund. After that, you can no longer get it. You can still file the return after that date.