The intermetallic calcium compounds CaTMg~2~ and CaTCd~2~ (T =Rh, Pd, Pt) were obtained by high-frequency melting of the elements in sealed niobium ampoules or through reactions in muffle furnaces. The polycrystalline samples were characterized by powder X-ray diffraction. They crystallize with a site occupancy variant of YPd~2~Si, a ternary ordered version of Fe~3~C. The structures of CaPdMg~2~ and CaPdCd~2~ were refined from single-crystal diffractometer data: Pnma, a=792.2(2), b=803.4(2), c=572.0(1) pm, wR~2~=0.0663, 1621 F^2^ values, 24 variables for Ca~0:94~PdMg~2:06~ and a=794.6(2), b=809.5(3), c=554.7(2) pm, wR~2~=0.0301, 819 F^2^ values, 23 variables for CaPdCd~2~. A small range of homogeneity was observed for Ca~1-x~PdMg~2+x~. The magnesium and cadmium atoms build up three-dimensional tetrahedral substructures (306 - 327 pm Mg-Mg and 307 - 317 pm Cd-Cd) that resemble hexagonal diamond, lonsdaleite. Together with the palladium atoms one obtains three-dimensional, covalently bonded [PdMg~2~] and [PdCd~2~] networks which leave cages for the calcium atoms. The latter are bonded to these networks via shorter Ca-Pd contacts (298 - 319 pm in Ca~0:94~PdMg~2:06~ and 295 - 312 pm in CaPdCd~2~). The course of the interatomic distances is in line with calculated overlap populations. The CaPdMg~2~, SrPdMg~2~ and CaRhIn~2~ structures are all derived from a CaIn~2~-related subcell by an ordered filling of transition metal atoms into trigonal prisms. This leads to different herringbone patterns for the networks of puckered and elongated hexagons of magnesium and indium atoms.