After you file

The program is quite clear what it is asking for ... it tells you how much of the credit you are eligible for and then asks you exactly how much you already got in advance and even gives links for more information if you are still unsure what you are being asked.  Your post seems to insinuate that you may not have known that the third stimulus payment you got was the third payment ... did you think it was the first ?  The first  round was $1200 single/$2400 for a couple + $500 per dependent under 17 listed on the 2019 or 2020 return ... total for you would be $2400 or $2900 if the child was under 17 at the end of 2020. 

Once you entered how much you got the next screen asks if you are very sure so I don't know how much more the TT DIY program can do to make this any more crystal clear but sadly you are not the only one who got this wrong ... the IRS is processing millions of returns with this error from all the DIY & professional programs alike.