Yes, you can amend your tax return to claim the 3rd Economic Impact payment only if you entered a "0" on your original return when you were asked if you received the 3rd Economic Impact payment.
In the Payments section, Line 15 check the box "Other" and enter RRC in the box. Then in Part III Explanations of Changes enter "Recovery Rebate Credit".
The instructions to Amend 1040 to claim the 3rd Economic Impact payment are under FAQ 1.
