Co(III) is hihgly  oxidant an oxidises water in solution. Its salts are inestable.. The E°pf the pair Co(III)-----Co(II) is very high

But Co(II) gives more complexes than Co(II)

Main cause that induce formation of binary compounds of Co(II) and very few for Co(III) is their specific electronic structure, namely for Co(III) formation of coordination compounds is more appropriate due to the d2sp3 hybridization in octahedral geometry.