I've never tried this in my own music room (never had more than a couple keyboards at once), but when I worked at a music store, we used Ultimate A-Frame tiered keyboard stands. I never liked the three-tier setup, because it made the middle keyboard too inaccessible, and the top keyboard too high up, and the bottom keyboard felt claustrophobic.
Of course, that was 11 years ago, and I'm sure there have been great leaps forward in keyboard stand technology. But side-by-side seems like a much better working system, if you have the floor space.