Why is the ripple frequency in dc side of 3 phase full wave converter is considered to be 6 times that of ac side?
Because there are 6 peaks per cycle in the combined input waveforms, and the three input sines are phased in such a way that none of these peaks coincide in time.

Think about it.  Each input sine has two peaks, one positive and one negative per cycle.  Three such sines therefore have a total of six peaks.  With the three input sines 120&deg; (1/3 cycle) apart, there will be one peak each 1/6 of a cycle, or every 60&deg;.

This is a rare case where a simulator might actually be useful. 

Simulation of 3 sines 120&deg; apart and its input and output waveforms.