The IUPAC name of $K_3\left[Co\right(C_2{O}_4{)}_3]$ is

Potassium tris(oxalate) cobaltate (III) 

Potassium tris(oxalate) cobalt (III)

Potassium tris (Oxalato)  cobaltate(III)

Potassium  trioxalato cobalt(III)