Yes, you should complete 2018 now and then you can download programs to complete prior years' returns. You do not have to complete then all at the same time.
Here's a link to the downloads for older years.
https://turbotax.intuit.com/personal-taxes/past-years-products/
I have tried to do the complete but it wants me to pay it again. [2018] and for 2015 it was paid as well.
@gomazcat1234- You seem confused. You can only use the current online software for a 2018 return. If you need to prepare a return for any prior year from 2015 to 2017, you must pay for the downloaded software to prepare the past year return.
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.