Simply put ... the CTC will be part of the 2021 return where it will be "reconciled" like the stimulus payments were on the 2020 return .... meaning what you got in advance reduces the credit on the return no matter how much you got.
So if the credit is $3000 and you only got $1000 in advance you will get the remaining $2000 on the return ... yes it is just that simple.
The third stimulus payment will also be reconciled on the 2021 return so if you did not get all of it in advance you get the portion you are eligible for on the 2021 return as a credit.
