first, the only thing that really matters is your 2020 tax return - both the income and the number of dependents. what happened in 2019 doesn't matter.
the 2020 tax return will calculate what you are due, subtract what you have already received and give you the difference. If that answer is less than zero, you will not have to pay back the remainder. Everything will appear on Line 30 of Form 1040.
if the 1st check was $1440, and I will assume it's a joint return with one child, then that would suggest your 2018 or 2019 income was $159,200. Again, it really doesn't matter because it's all going to be recalculated based on 2020 in any event.