Why does current flow from the most positive to the most negative in three phase system?
Think about two diodes feeding a load resistor. Don't think about 3-phase until after you understand this: -



If "A" is 10 Vots and "B" is 9 volts, "A" supplies all the current to the load. "B" can't supply any because its diode is reverse biased.

This is what happens in a 3 phase rectifier. Think about the scenario when the phase is about 130 degrees: both phase 1 and phase 2 will be at a positive voltage but only one of the phases will supply current to the load because its diode is the only forward biased diode. Same with negative sides of the voltage waveform - the most negative phase's diode will be the only one forward biased.