Why is benzene extraordinary stable though it contains 3 double bond .

Benzene is a hybrid of resonating structures given as:
      

All six carbon atoms in benzene are sp ² hybridized.
The two sp ² hybrid orbitals of each carbon atom overlap with the sp ² hybrid orbitals of adjacent carbon atoms to form six sigma bonds in the hexagonal plane.
The remaining sp ² hybrid orbital on each carbon atom overlaps with the s-orbital of hydrogen to form six sigma C–H bonds. The remaining unhybridized p-orbital of carbon atoms has the possibility of forming three π bonds by the lateral overlap of the C atoms.
The main reason for stability is:
The six π electrons are delocalized and can move freely about the six carbon nuclei. Hence, even after the presence of three double bonds, these delocalized π-electrons stabilize benzene to make it extraordinarily stable.