Now of course the number '3' is regarded as a magic number in myth and fantasy, and the notion of the trinity is of central importance in Christianity.

Watch this:

Take 3 entities, lets call them (E1,E2,E3)

Now pair them off. How many different pairings are there? (Ignoring the ordering). There are three. Look:

Pair 1: (E1, E2)
Pair 2: (E1, E3)
Pair 3: (E2, E3)

Now, for each pair, let one member act as the subject (or perceiver), and the other member act as the object (or thing perceived).

Now using all 3 pairs, each entity appears twice, once as a perceiver, and once as a thing perceived.

E1 -> E2, E1 -> E3
E2 -> E1, E2 -> E3
E3 -> E1, E3 -> E2

So the whole system is a circular loop, which needs nothing else outside itself - everything in the system is defined by something else in the system.

Father. Son. And holy spirit.