The three dependents must meet the requirements of a qualifying child.  See IRS FAQ question C1.

Those with qualifying children will receive up to an additional $500 per qualifying child. A qualifying child is a child who meets the conditions outlined on our Qualifying Child Requirements page.

Your question needs more details.  Are you claiming  3 Dependents in 2020?  Did you have a baby in 2020?

The Stimulus Checks are really based on your 2020 return we are filing in 2021.  So any amount you already got will be reconciled on your 2020 return line 30.  If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return.  But the good news is if they paid you too much they won't take it back.

No new baby just adding a 3rd dependent.

No no baby just adding a 3rd dependent.

If you have 3 qualifying dependents on your 2020 return, but only received stimulus payments for 2 of them, you will get the recovery rebate for the 3rd one.   In addition to being your qualifying dependent, they have to have a social security number (valid for work) and be 16 or under as of Dec 31, 2020.   

If one of the dependents you received a payment for turned 17 during 2020, they will not qualify you for the recovery rebate.