Current flows from a three-phase generator on the left, represented as three Y-connected single-phase sources, via a transmission line into a symmetric Y-connected load on the right. The phases have been arbitrarily coloured red, green, and blue. The angular separation between the phases is 120°, or 2π/3 radians. The supply frequency you observe in this animation will depend on your computer performance, but should be around 0.3 Hz.
At any point along the transmission line, the net flow is zero at all times during the cycle. The neutral circuit, which would connect between the centre Y-points of generator and load, has been omitted. In a balanced system, it would carry no current.
The phase sequence is red→green→blue, indicated by the order in which current flows into the three loads in turn.