How do I enter a lump sum disability payment that goes back 3 years?

You'll enter income (regardless of the time frame it covers) in the year in which the payment was received.  If you received a lump sum disability payment in 2018, that income will be entered in your 2018 return.  Any taxes owed will be due in the year in which the payment was received.

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Who paid you the disability pay? If it was the military veterans administration, then you won't receive any type of tax reporting document for it, and it's not reportable on any tax return.