Hybridisation of SO3

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Since SO3 has an expanded octet structure, the central S should form three double bonds with O, meaning S forms 3 pi bonds in total. However, after hybridisation of S, there are less than three unpaired electrons in unhybridised p-orbitals of S. How can S form 3 pi bonds when there aren’t enough electrons from unhybridised p-orbitals? Thanks.