I always try to use at least two keyboards. On house jobs I use one or two additional keys on my right hand at 90 degrees.
I had a three keyboard Yamaha Electone also. Neat thing is, you could "shake" your right hand while playing and get a response sensitive vibrato. The whole keyboard would move.
R.
I have toyed around with a 3rd key at 45 degrees to the right on top of my 2 mains, it actually worked out quite well at that time for solos...