The hybridisation of Manganese in KMnO4 is d3s. It is to understand that in coordination compounds the concept of hybridisation is slightly different from what we learn in chemical bonding.
In KMnO4, only three of the five molecular orbitals used for hybridisation are used for bonding which means that the four oxygen atoms contribute or donate electrons to 3 orbitals only.
In the given image shown below: