We have employed density functional theory calculations for determining intrinsic defect processes and structural, elastic, and electronic properties of recently synthesized Sn-containing 312 MAX phases M3SnC2 (M = Ti, Zr, Hf) including Debye temperature, Mulliken populations, theoretical hardness, charge density, and Fermi surface. The calculated lattice parameters justify the reliability of the present investigation, as they agree with the experimental values. The lattice constant a increases as the M-element moves from Ti to Hf in the periodic table. The mechanical stability of these compounds is verified with the computed single crystal elastic constants. Hf-based Hf3SnC2 is nearly isotropic elastically in view of the calculated parameters. The Debye temperatures decrease following the sequence of M-element: Ti → Zr → Hf. Zr3SnC2 and Hf3SnC2 should be better first coat thermal barrier coating (TBC) materials. The investigated band structures indicate that the electrical conduction increases as the M-element moves down from the top of the group in the periodic table. A gradual decrease in electronic density of states (DOS) at EF also follows the order of M-element in the periodic table. The covalency of M-C bonds is calculated to be increased as M-atoms moves from Ti to Hf via Zr. The rank of machinability for these compounds should be Zr3SnC2 > Hf3SnC2 > Ti3SnC2. The Fermi surface topologies of the three 312 MAX phases are almost similar and comparable with those of 211 MAX phase counterparts. Considering defect reaction energies, it can be concluded that Ti3SnC2 is predicted to be the most radiation-tolerant among Sn-MAX phases considered.