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, $Co$ is the cobalt metal, ${C}_{2}{O}_{4}^{2 -}$ are the ligands named as oxalato.

The charge on $[ Co ( {C}_{2}{O}_{4}{)}_{3} ]$ is $- 3$ and oxalate is having charge of $- 2$.

$X + 3 ( - 2 ) = - 3X = + 3$

Hence, the IUPAC name of the compound ${K}_{3} [ Co ( {C}_{2}{O}_{4}{)}_{3} ]$ is Potassium trioxalatocobaltate$( III )$.