So on your 2019 return your second child was only 4 months old? Did your 2019 return show the child as a qualifying child or just as a other dependent? When did you file 2019? If it was later they may have based your Stimulus Check on your 2018 return.
Make sure all 3 children show up as dependents and qualifying children. A child born (even on Dec 31) is considered to live with you the whole year. So pick the Whole Year at the top of the drop down box. Go back and check it under My Info. And check your 2019 return that you got credit for the one born in 2019 too.
