${\left[\mathrm{Co}{\left(\mathrm{en}\right)}_3\right]}^{3+}$ is more stable than that of the ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$.
This is because the chelating ligands form a more stable complex than the non-chelating ligands.
Since, ethylenediamine is a bidentate ligand and forms a stable chelate than that of the ammonia ligand.