EIP3 The 3rd Stimulus payment started going out in March 2021 and was for 1,400 for each person listed on your tax return, no age limit. The IRS used your 2020 (or 2019) return to send the checks out fast. If your 2021 return changes and you qualify for more you can claim the extra as the Recovery Rebate Credit on 1040 line 30. But line 30 is only if you didn't get the full amount or qualify for more. Not the amount you already got. The IRS also sent out letter 6475 to tell you how much they sent you . If you are married then each spouse should get a letter for half.
If you filed Joint with 2 dependents you should have received 5,600 (4x1400). So if your 2021 line 30 was zero or blank you must have answered that you got the full amount. . I would double check all your bank deposits in 2021 and Check your IRS account to see when and what they sent you (check both spouse's accounts if married). If your IRS account says 2,800 that was only half. Your spouse’s account should show the other half 2,800.
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...Your Online Account | Internal Revenue Service
