In ${\left[{\mathrm{MnCl}}_6\right]}^{3-}$oxidation state of manganese is $+3$, 

${\mathrm{Mn}}^{+3}$, electronic configuration is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^{4}4{\mathrm{s}}^0$, due to the presence of weak field ligand ${\mathrm{Cl}}^{-}$pairing of electrons do not occur, so the number of unpaired electrons are 4.

In ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$oxidation state of $\mathrm{Fe}$ is $+3$,

${\mathrm{Fe}}^{+3}$ electronic configuration is  $\left[\mathrm{Ar}\right]3d^{5}4s^0$

Due to the presence of strong field pairing of electrons takes place but one electron remains unpaired, $t_{2g}^5$ configuration is seen.

In${\left[\mathrm{Co}\left({\mathrm{C}}_2{\mathrm{O}}_4\right)\right]}^{3-},$ cobalt shows +3 oxidation state and electronic configuration, $\left[\mathrm{Ar}\right]3d^{6}4s^0$.

As ${\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}^{2-}$ behaves as a strong field ligand in complexes with cobalt, paring of electrons takes place and no unpaired electrons are left in this case.