Where on an amended tax return, can I add my missing Economic Impact Recovery stimulus payment 3?

When amending the return - Enter stimulus in the Search box located in the upper right of the screen.  Click on Jump to stimulus.

It will be reflected on line 15 of the 1040X :

why do you beleive you are missing EIP? what are the circumstances? are you adding a depedent that wasn't on your original tax return? did you enter when prompted that your received more in EIP than you acutally received ? (that would require an amendment to fix)

you were either eligible for EIP or the Recovery Rebate - not both. 

look at Line 30 of your tax return, is what you were due already there?

maybe an amendment is not necessary - trying to understand that before you go through more work than may be necessary