This will not take place on the 2020 tax return at this time, and maybe not at all. You should file your tax return as is with the original child tax credit (CTC), then the IRS will make the necessary payments separately for the third round of stimulus.
You could wait, however at this time I would advise you file your tax return and then wait for the IRS to release the funds. This way they will have the current information on how much you will be entitled to in full when they do the reconciliations. They are only paying an advance of part of the third stimulus right now.
Use the following links to stay up to date.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
