Question

Why did "d" orbitals start at "3d" instead of "4d"?

Answer 1

I think it is because of their energy level

Answer 2

I mean before 3d is 4s so why not 4d?

Answer 3

electrons are filled in increasing order of energies, in accordance to the aufbau principle.
According to it the filling of electron is governed by "n+l" rule and "n" rule.

for 3d orbital, n+l = 3+2 = 5
for 4s orbital, n+l = 4+0 = 4

since the n+l value of 4s is less than that of 3d , 4s is filled first.

See the atttachment for order of filling electrons.