It's the sum of all the three things, and then you divide by 3.

In mathematical notation, let's say you have things in x.

x_1, x_2, x_3, ... x_n

You have therefore n things. The average (or mean) is then:

\frac{x_1, x_2, x_3, ... x_n}{n} = \frac{\Sigma x}{n}

This is just some extra information :)