Four-electron transfer from U to the fullerene cage commonly exists in U@C-2n, (2n < 82) so far, while four- and three-electron transfers, which depend on the cage isomers, simultaneously occur in U@C-82. Herein, detailed quantum-chemicaI methods combined with statistical thermodynamic analysis were applied to deeply probe into U@C-84, which is detected in the mass spectra without any further exploration. With triplet ground states, novel isomers including isolated-pentagon-rule U@C-2(51579)-C-84 and U@D-2(51573) -C-84 as well as nonisolated-pentagon-rule U@C-5(51365)-C-84 were identified as thermodynamically optimal. Surprisingly, there were unexpected three-electron transfers, which directly led to one unpaired electron on the cage, in all of the three isomers. Significant covalent interactions between the cage and U successively weakened for U@D-2(51573)-C-84, U@C-2(51579)-C-84, and U@C-5(51365)-C-84. Besides, the IR absorption spectra were simulated as a reference for further structural identification in the experiment. Last but not least, the potential reaction sites were predicted to facilitate further functionalization and thus achieve promising applications for U@C-84.