You say your client did not receive the first EIP was it because the 2019 return was not filed when those payments were sent so IRS used the 2018 AGI to determine eligibility?        Waiting to file the 2020 so they will receive the $2400 for the EIP#1 is pointless...that shipped has sailed.   

If this third EIP is handled on the 2021 return as the EIP#1 & #2 is being handled on the 2020 return, where as a client does not have return any payment the exceeds what they are eligible for....waiting to file the 2020 until the EIP#3 of $2800 has been processed would be to your clients benefit.