Question: how many electrons are needed to fill sublevel 3s?
Answer: 1 atomic orbital in 3s sublevel x 2 electrons/orbital = 2 electrons can reside in the 3s sublevel. 3 atomic orbitals in 3p sublevel x 2 electrons/orbital = 6 electrons can reside in the 3p sublevel. 5 atomic orbitals in 5d sublevel x 2 electrons/orbital = 10 electrons can reside in the 3d sublevel.