Three phase AC system

As it is well known fact that in a DC system, two terminal exist
(positive and negative) and current or power flows from positive to
negative. so it can be say that one conductor carries current and
other returns the same current and there must be close loop to current
flow.

 I m wondering what happens to three phase AC system. In a 3 - phase
AC system, all three conductor carries current/ voltage/power with 120
deg apart assuming system is balanced. my question is how current is
returned to the source. let's say source generates power and power
flow from source to load but how about return power from load to
source?