The Recovery Rebate Credit amount is calculated based on your 2020 tax return. That credit is then reduced by any stimulus payments you received in rounds one and/or two (even if received in 2021).
Based on your post your credit amount would be: $2400 round one plus $1200 round two totaling $3600 for you and your spouse filing jointly. Your dependent is too old for the child payment.
If you selected that you did not receive any payments then your credit of $3600 is correct.
If you need to change to state you did receive payments just click on the "Federal Review" tab at the top of the page to re-enter the stimulus interview. Select "Yes" you received payments. Then enter the amounts in the appropriate box(es).
Your credit/refund will be adjusted.
Here is a link with more information on the Recovery Rebate Credit.
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