Syntheses and properties of low-valent clusters [X{Co(mu-CO)(PMe3)(2)}(3)], X = none (1), H (2), and H-3 (3), are reported. All solids are isostructural as explicitly shown by single-crystal structure data. The molecular structures of C21H54Co3O3P6 (1) and C21H57Co3O3P6 (3) contain central Cos units which form perfect equilateral triangles with Co-Co distances of 2.4055(5) and 2.432(1) Angstrom, respectively. Both compounds crystallize in trigonal space group R (3) over bar c, with Z=6: 1, a=10.678(1) Angstrom, c=52.298(11) Angstrom; 3, a=10.679(2) Angstrom, c=52.729(12) Angstrom. Compounds 1 and 2 form a continuous range of solid solutions. Powder samples exhibit molecular paramagnetism of different extent: At 298 K the effective magnetic moments for 1-3 are mu(eff)/mu(B) = 1.9, 3 1, and 2.7, respectively. The magnetic behavior could be rationalized in terms of the Curie-Weiss law for a one electron system (1) and two electron system(2). The two electron approximation has been discussed as well for 3 above 6.2 K. A description of the scope of exchange coupling is presented for 1.