The EIP3 would have nothing to do with your 2020 tax return. The EIP3 payment was issued in 2021 sometime in late February to March. In 2020, there were two other stimulus payments issued and that is probably what you are seeing on your 2020 account.
Before you claim the $1,400, check your bank account and make sure that you did not get a deposit from the IRS sometime in late February to March 2021 in the amount of $1,400. I would also look for a deposit in the amount of $1,400 during the same time period in case you received a check instead of a direct deposit.
Once you have verified all of this information, then you could claim the credit and Yes it may delay the processing of your return. Keep in mind if you are convinced that you did not get the EIP3 then the only way you will get it is to claim it.
Is there any possibility that it could have gone to another account that may have been closed? If you had a direct deposit of a refund on your 2019 taxes, double-check the account that refund would have gone. That is the account that most likely EIP3 would have gone.
