Orthophosphoric acid $(H_3PO_4)$ have the following structure It is clear from the structure that it contains three replaceable hydrogen atoms, so it gives three H+ ions on dissolution in water. So, the basicity of $H_3PO_4$ is three $\, \, \, \, \, \, \, H_3PO_4 \, \longrightarrow\, 3H^+ + PO^{3-}_4 $
P block elements are those in which the last electron enters any of the three p-orbitals of their respective shells. Since a p-subshell has three degenerate p-orbitals each of which can accommodate two electrons, therefore in all there are six groups of p-block elements.
P block elements are shiny and usually a good conductor of electricity and heat as they have a tendency to lose an electron. You will find some amazing properties of elements in a P-block element like gallium. It’s a metal that can melt in the palm of your hand. Silicon is also one of the most important metalloids of the p-block group as it is an important component of glass.