This sounds like a question about electron energy levels. The third principal energy level has three sublevels, s,p and d.
This means that it can have a maximum of 18 electrons. Read on to learn why:
The sublevels possilbe for each principal energy level are as follows:
n=2: s and p
n=3: s, p and d
n=4 and higher: s, p, d and f
The sublevels have various numbers of orbitals, which are regions of probability of finding an electron, and each orbital can have a maximum of two electrons.
Here are the number of orbitals and maximum number of electrons possible for each sublevel:
s-sublevel = 1 orbital = 2 electrons
p-sublevel = 3 orbitals = 6 electrons
d-sublevel = 5 orbitals = 10 electrons
f-sublevel = 7 orbitals = 14 electrons