You should know you have to report all your income. Did you get a W2? Don't know how you skipped that part. And the 3rd Stimulus payment was not just for married with children. Single can get it, even with no income. Don't know what you're thinking of. Same for the EIC earned income credit.
The 3rd Stimulus payment started going out in March 2021 and is 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.
