Question

why d3sp2 hybridization not possible

Answer 1

For d3sp2 you are going to have 6 ligands around central atom .
The possible geometries in such a case are ,in bracket symmetry point groups are indicated.
a)Octahedral(Oh)
b)Trigonal prisamatic (eclipsed D3h / staggered D3d)
c)plane hexagon(D6h)
because if one wants to use set of three orbitals a)of same shape ,it would be d(xz), d(yz),d(xy)
b)otherwise one of either d(z2)/d(x2-y2) &
two from d(xz),d(yz),d(xy).
one s
two from p(x),p(y), p(z)
Will combination ‘b’ be really possible resulting in some geometrically shape .(I doubt.)
you may think of somehing innovative.