You said, "If I purchase a past year tax return from TurboTax can I also do the rest of my taxes..."
It's not clear what that means. Do you need to prepare 2016 and 2017 returns, or are you trying to pay to retrieve previously-filed 2016 and 2017 returns that are locked in your online account?
In other words, are you trying to download and print 2016 and 2017 returns that you previously filed but whose access is locked? Or are you asking about purchasing 2016 and 2017 desktop products (downloads) to prepare a 2016 return and a 2017 return?
I have to do a tax return for 2016 and 2017 and I would like to once I purchase to do so can I use to do both years
Each tax year has its own specific software. You cannot use the same software for more than one tax 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)
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.
If you choose to use TurboTax to complete and file the 2016 and 2017 tax returns, then you will need to purchase, download and install on a personal computer one of the desktop editions for each tax year from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
Prior year tax returns can only be printed and mailed, they cannot be e-filed.
When mailing multiple tax years use a separate envelope for each tax year. Also consider using a tracking service return receipt required from the US post office to track the mailed returns and have proof the IRS received the tax returns.