No one knows. There has not been a third stimulus passed by Congress. We do not know if or when it will happen. And we really really do not know how the IRS will have to handle it if they do it in mid tax season.
How much of a delay do you believe is going to be expected if you file taxes normally this year?
So far, the IRS has delayed the start of tax processing until February 12.
If Congress passes a new stimulus that is added to the 2020 tax return, or if Congress forces the IRS to make a third round of payments instead of processing tax returns, who know how long it will take. The IRS computer system is 50 years old and every attempt to update it in the past has ended in bureaucratic failure. (Do you know anyone who can program in COBOL? They can probably get a good job in Washington this year.)
I would guess that if Congress passes a new stimulus that must be paid now, or can be claimed on a 2020 tax return, it will shut down all tax return processing for at least a month from that date, and delay refund payments indefinitely for anyone who filed before the approval of a third stimulus. Hopefully if there is a third stimulus it will at least be part of 2021's tax return and not 2020.