can hybridisation can be of type d³s

yes there can be hybridisation of d^3s type.but in this the ns orbital will combine with (n-1)d orbital and not with the same shell.the reason is the large difference in energy and secondly the in between s and d orbital there is a p orbital which intereferes in their hybridisation.

in MnO^4- and CrO4^2- the hybridisation is most probably d3s