It sounds like you already received the 3rd EIP payment for yourselves and one dependent. 3 x $1400 = $4200 and do does 2 x $2100. You cannot get the 3rd EIP again. You must enter that you each received $2100.
When you enter the amounts you already received that should leave line 30 of your Form 1040 blank---and it should be blank unless you had a baby or added some other dependent during 2021.
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