If you purchased crypto currency in 2017 and 2018, held them and then sold them in 2020, you would enter the sale in 2020 and enter the date your acquired them so the loss on sale would be a long term capital loss in 2020. I assume by losses you mean the investments went down in value, but you don't report that on your tax return until you sell or trade them, or when they become worthless if that happened.
You should wait until your 2017 return is processed before you amend it. Otherwise, the amended return might not be accepted by the IRS.
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