They would not have received anything in 2020 or 2021, since the filings for 2020 and 2021 were done by this person (nothing upon which to base the advance payout for RRC/EIP). There is no indication if the 2021 filing was on time or late, but at the least, there seems to have been someone qualifying in the household for some of the credits and maybe for some of the years or all of the years.

If you want to debate the IRS, you're likely going to have to break it out by taxpayer and dependent by tax year and by available credit(s) to figure out which amount(s) to which they are entitled. The eligibility for EIP 3 is different than for EIP 1 and 2. No one without access to every fact and detail will be able to do this.