What is the basicity of H₃PO₄?

The structure of H₃PO₄ is tetrahedral. It has three P—OH and one P = O bond as                

Shown below: 

Since it has three ionizable P—OH bonds, therefore, H₃PO₄ is tribasic as shown below: 

Since it is tribasic, it forms three series of salts with NaOH, viz., NaH₂PO₄, Na₂HPO₄ and Na₃PO₄.