You are correct. The calculations are correct. The Recovery Rebate Credit is based on the information in your 2020 tax return. The stimulus payments received in rounds one and/or two were advances on that credit. At the time of stimulus rounds one and two, a 2020 tax return was not available so prior year return information was used.
On your tax return you would be eligible for you and 2 dependents. That would be 1200 plus 1000 (round one) and then 1800 (round two) for a total of $4000. That amount would be reduced by the stimulus payments you actually received in rounds one and/or two (2700 and 2400 totaling $5100). There would be no credit left to add to your tax return.
Here is a link with more information on the Recovery Rebate Credit.