You can only do 2019 online. Since you didn't file 2018 then enter 0 for the AGI to efile 2019.
Sorry, Online for 2018 and prior is closed now so you can not start a new 2018 return or finish one. Online is only for 2019. Oct 15, 2019 was the last day to efile. 2018 is now the prior year.
Did you start an Online return? How to finish a prior year online return
Or How to file a prior year
You can only buy the download for prior years. You have to buy a separate program for each year. They only sell the last 3 years. You need a full Windows or Mac computer to install it on. Buy the download here
You have to print and mail prior year returns. 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. Don’t forget state.
Important! Mail Federal and State in separate envelopes and mail each year in separate envelopes.
If you are getting a refund, there is not a penalty for filing past the deadline. You have 3 years to file for a refund. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.
Or If you have a simple return You can fill out the blank forms for free.
Here is the IRS 2018 Form 1040
And 2018 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf
IRS 2018 Schedules 1 thru 6 (only if needed)
Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf
Sch 4 : https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf
Sch 5 : https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf
Sch 6 : https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf
And I have the links for other years too.
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Exactly what do you mean when you say you "have filed three years of taxes." What did you do? Each tax year is separate. You cannot combine them and put income from more than one tax year on the same tax return--did you do that? Did you already try to e-file? What happened ---was it rejected? We can help---and it is much easier to prevent mistakes before you file a tax return than it is to fix them if you file incorrectly.