i use 2 cds decks and 2 vinyl and i usually use the other ones that arnt in the imediate mix to cue up the 3rd or 4th record im gonna play etc.
can leave loads of time for adding effects ad other things, beats standing around aswell, might aswell plan ahead, its all about planning.
cant say id ever chuck 3 in at once, can be too much unless your using minimal genre of music,

i do use 3 rd deck for adding vocals etc, but not too much, gets messy.