In $\left[ MnCl _4\right]^2: d^5$ configuration and $5$ unpaired electrons

In $\left[ CoCl _4\right]^2: d^7$ configuration and $3$ unpaired electrons

In $\left[ Fe ( CN )_6\right]^4: d ^6$ configuration, low spin complex and o unpaired electrons

More the number of unpaired electrons, more the value of magnetic moment.

Hence, the order is $\left[ MnCl _4\right]^{2-} > \left[ CoCl _4\right]^{2-} > \left[ Fe ( CN )_6\right]^{4-}$