The electronic structure of HgBa₂Ca₂Cu₃O_{8+ delta} with various delta (0, 0.12, 0.2 and 0.4) and pressures (ambient pressure, 5 GPa, 10 GPa and 20 GPa) has been calculated by the recursion method. Our results show that the electronic structure of this series is highly two dimensional with a large N(E_F). The hole concentration of the CuO₂ layers and N (E_F) increase monotonically as delta increases from 0 to 0.4. We thus suggest that the ideal delta should be 0.4. With increasing pressure, the hole concentration increases further and E_F rises. The relationship between T_c and pressure can be described by the same model of YBa₂Cu₃O_{7- delta}.