Assuming that you are married and filing Jointly, you were supposed to indicate the total of amount of the 3rd stimulus payments received by both you and your spouse, $4,200. You both should have received IRS Letter 6475 showing the amounts you each received, $2,100 each. If you only entered the amount that you received, $2,100, then on your federal tax return, Form 1040, the Recovery Rebate Credit of $2,100 was entered on Line 30 reporting the amount that you did Not receive. The IRS has records showing that the 3rd stimulus payments of $2,100 was sent both to you and your spouse.
Go to this IRS website for your tax account to see if the 3rd stimulus payment is entered - https://www.irs.gov/payments/your-online-account
If you are Married Filing Jointly also check your spouse's tax account.
If you checked your bank account in the March - May 2021 timeframe and there is not a $4,200 EIP3 payment from the IRS then go to this IRS website for how to start a trace on the payment - https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
