I'm pretty sure the majority of guys running 3 axle trailers are most often running a tandem, stock walking beam spring set-up on two axles, and then an individual leaf spring set-up for the third. Others may not run any springs at all, the third axle is a lift axle. They use a servo to raise and lower it, sometimes with, and sometimes without a single leaf spring.
Alternatively, you could run a simulated air bag set-up, available from PMD. Then every axle would be sprung individually, with no walking beams and no leaf springs.
Another good idea (my personal choice) would be to run two walking beam spring packs, across the three axles, with the center axle being sprung equally by the two walking beam spring packs. The outer axles, simply having one end each, of the spring pack. Performance-wise, that would probably give you the best suspension. I have no photos, but it's not to complex. Perhaps others will chime in with some of their set-ups.
This happens to be a perfect example of a three axle, walking beam suspension, I just found. Look at Kyle's pictures and you'll find exactly what you need.
http://rctruckandconstruction.com/sh...96&postcount=4