I mistakenly listed I didnt receive my 3rd stimulus when I did.How do I fix this? Will I get fined for the mistake?

It depends. If you have already filed your return and it has been accepted then you don't need to do anything. Do not file an amended return since the IRS will adjust your tax return on their own end and will send you the adjusted refund amount. You will not be fined for this mistake so no need to worry about it.

If you have not filed your return or if your return gets rejected, then you can simply go back to your return and edit your response and file it again.

However, there is no need to amend the return and even if your return gets accepted, the IRS will make the adjustments on their own end.