The IUPAC name of K 3 [ Co ( C 2 O 4 ) 3 ] is:

Ligands that include a numerical prefix in the name use the prefixes bis for $2$, tris for $3$, or tetrakis for $4$ to indicate their number.

If the complex ion is an anion, we drop the ending of the metal name and add –ate.

In the given coordination compound, $\mathrm{Co}$ is the cobalt metal, ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ are the ligands named as oxalato.

The charge on $\left[\mathrm{Co}\right({\mathrm{C}}_2{\mathrm{O}}_4{)}_3]$ is $-3$ and oxalate is having charge of $-2$.

$$\begin{gathered} \mathrm{X}+3(-2)=-3 \\ \mathrm{X}=+3 \end{gathered}$$

Hence, the IUPAC name of the compound ${\mathrm{K}}_3\left[\mathrm{Co}\right({\mathrm{C}}_2{\mathrm{O}}_4{)}_3]$ is Potassium trioxalatocobaltate$\left(\mathrm{III}\right)$.