I'm trying to file for the last three years and it's putting them in as one year. How do I separate them?

You cannot put three years of income on the same tax return!  That would be  a huge mistake.   The current online software is ONLY for 2018.  If you have to file for other past years you need to use the correct software.  Delete anything you have entered that is not for 2018!

Online preparation and e-filing for 2017 is permanently closed.

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 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.