The A&B inputs let you merge, do houselights or failover. Imagine a 96 way
dimmer pack with 72 dimmers as the lighting rig and 24 as the
house lights,
fed from two different controllers
etc.
As for the 3-pin vs 5-pin. 5 pin is the actual standard and is found on most pro gear. 3 pin was done on low end budget gear so folks could use
microphone cables (even though 3 pin data is different from 3 pin
microphone cable.)