.....the more sounds you layer, the closer you get to a farfisa-like organ timbre 
.......OR WORSE!
I think 3 is plenty, but that's a subjective opinion, from a non Farfisa fan.
 I agree, WHO DOES THAT?  If PERFECTLY MATCHED, the piano/strings thing is about the only thing that seems to have stuck around and even that is an amateurish, over-used, 'poor man's' way of trying to create a little musical interest and is NEVER, EVER used in a professional production.  Farfisa's evolved from accord...WHOOPS, sorry, but they obviously were invented by someone who hated music 

.  There's a 'FARFISA' setting on my Nord organ....I always wondered why 

.  Actually, I'd like to layer a Farfisa, a Bagpipe, and a sampled violin played by a six-yr-old, and see how long it would take to drive someone crazy.  Yep, three layers is definitely enough...IMO.
chas