How many electrons may be accommodated in the first three energy levels?

Each shell can contain only a fixed number of electrons: The first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on. Some points will be nice

The first energy level can hold 2 electrons after that all other energy levels hold 8 electrons which means 3 energy levels can hold 18 electrons 2+8+8=18

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.