How can I file last year an the previous years return together?

[MinhT]

Tax works by calendar year. You cannot combine two years together.

[xmasbaby0]

You cannot do that.  Each tax year has to be filed separately using the forms and software for that specific tax year.

To file a return for a prior tax year

Only a 2017 return can be prepared online and e-filed. 

 If you need to prepare a return for 2014, 2015, or 2016 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.