If you were claimed as a dependent on a 2020 return then you should NOT have gotten any of the advance third stimulus payment paid to you directly ( whoever claimed you on the 2020 return go the advance based on how many people were listed on their 2020 tax return) so you will also NOT get a 6475 letter saying you got an advance you were not entitled to or was never sent directly to you.
On the 2021 return, if you are not going to be claimed as a dependent, when asked how much you got in advance simply enter zero and if you now qualify for some or all of the credit it will show up on line 30 of the form 1040.
