This is a formally $\ce{Mn(VII)}$ compound and hence there are no $3d$ electrons. 

The four $\ce{O^2-}$ ions are considered to be donating two electrons each to the $\ce{Mn}$ Atomic Orbitals. Now most chemists didn't realize that tetrahedral "hybridization" can be achieved by using the $4s,~3dxy,~3dyz$, and $3dxz$ AOs. Since in $\ce{Mn(VII)}$ the $3d$ AOs are lower in energy than the 4s and 4p AOs the "hybridization" is best considered as $d^3s$. I have put "hybridization" in quotes because it is not used in advanced chemistry (it is an artifact!) but it is correct in saying that the bonding MOs contain mostly $4s,~3dxy,~3dyz,~ 3dxz$ character but they'll also probably mix some 4p character in there for good measure!