will I have to also complete the 2017-2019 returns in TTX as they were originally filed then do the amended returns?
Yes. Obviously, you'll start with the 2017 return first. When you complete the 2017 tax return you'll have to elect the option to "Print and Mail" in order for the program to "know" it's already filed. After you print it, compare the new printout to the original printout and make sure things match exactly before you start the amend process. This is important, because the critical program updates for TTX 2017 are no longer available, as TurboTax only supports their program for the current tax filing year and 3 years back. That's it. So if things don't match, either you re-entered something wrong or a critical update is needed - which you can't get. Don't know what you'll do if it's a needed update thing; other than have a professional take care of it for you.
Meanwhile, if you haven't done so already you may want to go ahead and file an extension for your 2020 taxes since it's unlikely you'll make the May 17 filing deadline.
Take note also that for the amended returns, you'll have to print, sign and mail them most likely. While you can e-file an amended 2020 tax return, I don't think you can do so with a prior year tax return.
Make sure you mail each tax year in a physically separate envelope to the address on the printed mailing instructions for that tax year also.
Finally, if a state tax return is involved here be aware that a separate page of instructions for the state return will be printed.
