The third stimulus payment is based off the 2019 return OR a 2020 return that was fully processed before they started sending out the payment. This also happened with the first 2 payments last year ... so if you do not get the rest of the payments you are entitled to in advance then you will be able to apply for the missing money as a credit on the 2021 return you file next year. Sadly the system is not perfect and never will be.
