You can amend your return to change your filing status from Married Filing Separately to Married Filing Jointly. Initially, based on the already filed returns you will be ineligible for the third round of stimulus payments. The verbiage in the new law indicates if an amendment changes the eligibility status from ineligible to eligible the IRS will issue the payment, eventually.
The amendment forms are projected to be available in TurboTax around March 25. You should wait for your original return to be processed. Then sign in to your TurboTax account and select the "Amend" option. Follow the prompts to complete your amendments. You will need to print, sign and mail in the amended returns.
Married Filing Jointly will most likely have other benefits as well, like lower tax rates and eligibility for some credits and deductions. Here is a link with discussion on your filing status.
Here is a link to monitor for updates on the stimulus payments.
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