IRS will "top off" EIP#3 if the 2020 return shows the taxpayer eligible for more than what was paid before it was processed.  That's if the return is processed by August 15 (90 days after the new due date of May 17)  They will not recompute EIP#1 or EIP#2.  You do those on the 2020 return, using the RRC worksheet, where you enter zero if nothing was received.