Can I cancel my taxes and refile? I accidentally filed 2017/2018 taxes as my 2019 which I'm not filing at this time. Help! Is there any way to fix this?

You now have a problem.  You have to file each year separately.  Online is only for 2019.  You have to wait and see if your efile gets accepted.   If they reject it for something you can change it around to be only for 2019

But if they accept it you need to wait some more for it to fully process and you get the first refund.  Then you need to Mail in an Amended 1040X return to change it to be for 2019.

You can check your efile status here   https://turbotax.intuit.com/tax-tools/efile-status-lookup/

How to amend an Online return

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/

Amended returns can only be MAILED not efiled. 

Then if you still need to do 2018.  To do 2018 you have to buy the Desktop Download program here

https://turbotax.intuit.com/personal-taxes/past-years-products/     

Or If you have a simple return You can fill out the blank forms for free.

Here is the IRS 2018 Form 1040

https://www.irs.gov/pub/irs-prior/f1040--2018.pdf

And 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)

https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf

https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf

https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf

https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf

https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf

https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf

Don’t forget your state.  And you will have to print and mail your returns.  So be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.

Once a tax return has been e-filed it cannot be canceled, retrieved or changed.
If you have e-filed your tax return and need to change, add or delete information on the tax return then you will need to wait for the tax return to either be accepted or rejected by the IRS.
If the tax return is rejected then you can make your changes and re-file the tax return.
If the tax return is accepted then you will have to amend the tax return to make your changes. An amended tax return, Form 1040X, can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return. Wait until you have received the tax refund or the taxes owed have been paid and processed by the IRS before amending the tax return.

To access your online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Amend (change) return

See this TurvrboTax support FAQ for amending a 2019 tax return - https://ttlc.intuit.com/community/tax-topics/help/how-do-i-amend-a-2019-return-in-turbotax/01/1238456

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To complete a 2017 and 2018 tax return using TurboTax you will need to purchase, download and install on a personal computer one of the 2017 and 2018 desktop editions 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.  Use separate envelopes when mailing multiple tax years.