well i think its more like 3 and four channel abs.....
well 3 point/channel abs is mostly common in American trucks(rwd), there are three sensors, 2 monitoring the front wheels and one monitoring the propeller shaft/diff.. in this configuration there is a possibility that one of rear two tires locks up...
4 channel has sensor in all four wheels...
brake assist basically sends different amount of hydrolic pressure to different tires, depending on the loading of the car..