In order to elucidate the features of the second hyperpolarizabilities (gamma) for n-center radical systems (n, integer), we investigate the gamma of symmetric one-center radical models, i.e., BH3(-), CH3, and NH3(+) and three-center radical models, i.e., BH(CH2)(2)(-), CH(CH2)(2), and NH(CH2)(2)(+), by various electron-correlation and a density functional methods using extended basis sets. Since these models are isoelectronic systems involving 9 and 23 electrons, respectively, the gamma values are expected to reflect the features of their charge density distributions. It is actually found that the gamma values for the isoelectronic systems sensitively depend on their charged states. For instance, the one-center radicals exhibit positive gamma values at high-order electron correlation level using sufficiently large basis sets, while, for the three-center radicals, only NH(CH2)(2)(+) is found to possess negative gamma. These features are analyzed using gamma density plots, which can provide a local view of spatial contributions of electrons to the gamma, and are discussed in relation to our structure-property correlation rule.