You can only mail in prior year amendments.
I suggest you wait for the 2018 amended return to be processed before sending in your 2019 amended return. Likewise, wait for the 2019 amended return to be processed before sending in the 2020 amended return. I don't know the specifics of what you're amending but it may affect subsequent years. Another reason to wait is to make sure the IRS allows the amendments.
For tax year 2019, since you itemized, the state refund should be taxable on the federal return. You want to make sure you get the refund. If you enter the 2019 amended state refund on your 2020 amended return but the amount is different or not allowed, you'll have to amend your amended return.
Additional information: How to amend (change or correct) a return you've already filed?
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