Question

What does it mean when they say "Events happen in a "series of three""

Answer 1

It is superstition that good (bad) things happen in groups of three. Sloppy counting probably explains this.

Answer 2

This means that they are in the same sequence as some other named item.

For example, you might say my first three choices are red, green, and blue, respectively. This implies that red is your first choice, green is your second choice, and blue is your third choice.